For the past 40 years or so very nearly all of the small marine diesel engines of less than about 50hp have been of the IDI pre-combustion chamber type that does not work well at all. The few small direct injection diesel engines that were available have long since been discontinued, leaving the 35hp Yanmar 3JH as the smallest direct injection marine diesel engine.
What is Small?
The Detroit Diesel
The Volvo Era
A Sabb Story
Engines Marinized by BMW
Yanmar Goes Direct Injection in a Big Way
The Current Situation
Examples of Indirect Injection Diesels
Air Cooled Diesel Engines
The China Diesel
For quite a few decades the Perkins 4-236 was widely considered the smallest diesel engine available, and along with the very similar Perkins 6-354, was about the only game in town for a direct injection engine. In the 1980's the Cummins 6B and 4B came on the scene providing a similar size engine for the same types of applications. The Perkins 4-236 and 6-354 engines are generally considered to be the last engines ever offered with regulator valve equipped rotary distributor injection pumps, but into the 21st century there were some Cummins 6B marine propulsion and generator engines available with fully mechanical rotary distributor injection pumps. Throughout the later decades of the 20th century there was also the inline injected six cylinder Ford Lehman marine diesel which was about the same size as the Perkins 6-354 and the Cummins 6B. These were the small engines, but small is relative. The Perkins 4-236 marine engine was known as the M90, indicating that maximum power output was 90hp. The output rating at the propeller shaft was 87hp at 2800RPM, but the big heavy pistons and rods on the five inch stroke really worked much better down around 1500 to 2000RPM and the normally aspirated 4-236 made nearly it's full rated power output by 2400RRPM. With the flexibility of the regulator valve equipped rotary distributor injection pump good efficiency was attainable over a fairly wide range of engine speeds and loads. From about 2000RPM down to 1500RPM efficiencies of 155 to 160g/hphr were attainable under medium to heavy loads, and all the way down to 1200RPM lighter loads were able to be suported quite well. The Perkins 4-236 had a reputation for idling along on very small amounts of fuel, but it also had a reputation for spectacular failure when somenthing went wrong with the poorly understood injection system. The four cylinder Cummins 4B engines were not very popular, but the 6B was widely used in all sorts of small vessels. With a rotary distributor injection pump even 200 or 300hp turbocharged 6BT engines could run quite well all the way down to one gallon per hour at 1200RPM, which allowed fast planing boats to idle along at hull speed descently efficiently.
The Detroit diesels were produced by General Motors for a large part of the 20th century and were widely known as GMC diesels or "jimmies". They are two stroke diesel engines that use unit injectors and run in the neighborhood of 1200 to 2300RPM. The intake ports are in the cylinder walls and four in-head exhaust valves per cylinder are driven by a camshaft. Intake air is forced in at up to about 3psi by a gear driven roots type compressor. The original 71 series engines used a five inch stroke, and as the name implies were 71 cubic inches (1162cc) per cylinder. In 1957 the smaller 53 series engines were introduced but the shorter four and a half inch stroke meant that they had to run eleven percent faster so they were not all that much smaller really than the 71 series engines. There were a large number of 2-71 engines produced, and these are essentially the smallest of the Detroit Diesels. Stories of 1-71 engines and 2-53 engines abound, but the reality was that the 2-53 was not produced in large numbers and even the 3-53 was quite rare. What were used in large numbers were the 4-53 and the 2-71, with the 2-72 being considerably smaller in terms of output and fuel consumption and the 4-53 being a compact high output engine better suited to use with complex transmissions. Being two stroke engines even the 142 cubic inch 2-71 is a rather large engine and is rated at up to 68hp at 2100RPM. With the unit injection system though these engines worked fairly well over a range of engine speeds, and injectors with different injection flow rates were available to match the engine to the specific application it was being used in. With a lower injection flow rate the 2-71 was used as a 1200RPM generator set engine for 20kW output. With 1.5GPH fuel consumption under a full load the gen set engine runs quite efficiently at about 30hp output but minimum fuel consumption remains rather high at more than a half a gallon per hour. As well as the lower injection flow rate works for 30hp output the big 2.3 liter two stroke (roughly equivalent to a 4.6 liter four stroke) needs to make more power to run at it's best over a wide range of engine speeds and loads. The Detroit Diesels did do well with making rather large amounts of power up at the high end of their engine speed range, and were in a time dominated by much slower diesel engines known as "screaming jimmies". The characteristic screaming sound of the Detroit Diesels was the result of a high injection flow rate and early opening exhaust valves. The Detroit Diesel most widely used in small boats was the 230hp 6-71, not a small engine by any stretch of the imagination.
Starting in the 1960's Volvo began producing a line of one and two cylinder inline injected direct injection marine diesel engines. These 3.54 inch stroke engines at first were setup to run best at about 1500 to 2000RPM, but this was a bit too slow of a piston speed and peak efficiency was a slightly disappointing 190g/hphr at 1200 to 1700RPM with smoky operation. Maximum output was 7hp at 2300RPM for the single cylinder MD1 and 15.5hp at 2300RPM for the two cylinder MD2 with a displacement of 445cc per cylinder. By the 1970's the Volvo MD series engines had gotten lighter and were making more power with lower emissions, but peak efficiency improved only slightly and light load operation got even worse. The higher output and lower emissions were due to the injection systems being setup to run at higher engine speeds, but with the inline type pumps higher speeds meant less flexibility and poorer light load operation. The 1970 to 1975 MD2B was punched out to 1120cc and power output was all the way up to 25hp at 2500RPM with a peak efficiency of 177g/hphr from 1500 to 1900RPM. The MD2B from the 1970's was widely considered to be the best of the small Volvo Marine diesel engines. The single cylinder model did not work as well because the accessories, including the transmission, were large, power hungry and the same pieces as on the two cylinder engine. Notable Volvo models from the late 1970's were the 45 degree inclined single cylinder MD5 and the two cylinder MD11C with it's bigger three cylinder sibling, the MD17C. The little 3.15 inch stroke MD5 was quite compact at less than 20 inches high, and obviously intended for installation under low cockpits in small sailboats. The performance of the 7.5hp rated 443cc MD5 was however a bit lackluster. The main problem with the MD5 seems to be the short 3.15 inch stroke and an injection system setup for very low speed operation. Maximum engine speed was 2500RPM and peak efficiency under a full load came all the way down at 1700 to 1900RPM where a rather poor 215g/hphr was delivered. The MD11C with it's 3.54 inch stroke, 1120cc displacement and 2500RPM maximum engine speed was nearly identical to the MD2B it replaced, but peak efficiency under a full load was very slightly better with 175g/hphr available from 1600 to 1900RPM. Interestingly the 35hp three cylinder MD17C attained the same 175g/hphr peak efficiency under a full load at 1700 to 2100RPM. A wide variety of other models were introduced and quickly discontinued throughout the 1970's and into the 1980's culminating with the 2000 series engines introduced in 1983. The general trend was towards even higher engine speeds, and the stroke of the 2000 series engines was actually reduced to 3.4 inches. The three cylinder 2003 worked better than the two cylinder 2002 for the same reasons that the old MD2 had worked better than the MD1 (oversized accesories). With the 2000 series engines though the old problems of limited ranges of speeds and loads were even more accentuated since they turned so much faster. Displacement was down a bit, with the 2002 coming in at just 850cc, but power output was still high with 18hp at 3200RPM. Peak efficiency for the two cylinder dropped off to 190g/hphr at 2300 to 2700RPM but the three cylinder was able to attain a peak efficiency of 173g/hphr from 2100 to 2500RPM and 180g/hphr at the rated 28hp output at 3200RPM. The single cylinder 2001 had a much worse peak efficiency with 205g/hphr from 2200 to 2500RPM and 221g/hphr at the rated 9hp output. The biggest problem with the 2000 series engines was clanking at low engine speed and an inability to run well under a light load but they also had a reputation for being expensive to own and they were discontinued in the early 1990's.
Most of the old Norwegian Sabb life boat engines were of the IDI pre-combustion chamber type, but a direct injection engine rated at 30hp was also produced. This big four and three quarter inch stroke 1880cc two cylinder engine was rated at 30hp continuous at 1900RPM. People who have used them say that they work quite well, but 30hp continuous is not really all that small of an engine. The 10hp single cylinder Sabb engines displacing 760cc with the same long four and three quarter inch stroke were of the IDI pre-combustion chamber type and were not nearly as efficient. Still though the long stroke and rugged design made them popular power plants for extended use.
In the early 1980's BMW introduced a marinized version of the water cooled single cylinder Hatz direct injection engine of that time. This little 280cc industrial engine was rated by BMW for marine use at 6hp at 3600RPM. With just a 2.83 inch stroke the little BMW with it's metering collar type injection pump was setup to run quite fast. Even low idle below 1500RPM did not work well, and maximum torque came all the way up at 3300RPM where the engine was able to make 5.5hp. Specifications for efficiency were not available for this engine, but people who used them said that they did very well on fuel near maximum output. The little BMW was available until BMW Marine Gmbh wend out of business in 1987. Before closing shop though larger and better functioning direct injection engines were added to the model lineup. These larger engines were based on a series of two and three cylinder water cooled direct injection industrial engines also manufactured by Hatz. With a stroke of nearly four inches these engines ran much better at lower engine speeds. The 1416cc D35 was rated at 30hp and the 2124cc D50 at 45hp continuous output at 3000RPM. The D50 could do 170g/hphr at 2500RPM. Although these bigger engines worked quite well the range of speeds and loads they could operate over was severely limited by the inline injection pump and long high pressure lines.
The same little 280cc direct injection Hatz diesel was also marinized by Renault and carried a maximum output rating of 8hp at 3900RPM. Renault and BMW both also marinized a 12hp two cylinder IDI engine from Hatz that incidentaly was one of the better IDI marine engines ever available. With the same 3.94 inch stroke as the big two cylinder direct injection D35 but a smaller bore for a total displacement of 528cc the 10hp continious rated engine was able to attain 200g/hphr at 1800 to 2200RPM.
The smallest direct injection Yanmar marine diesel in the 1980's was the 3.4 inch stroke 1644cc four cylinder 4JH rated at 40hp continuous at 3500RPM normally aspirated and 50hp continuous at 3500RPM turbocharged. With peak efficiency of 175g/hphr at 2500 to 2800RPM and 183g/hphr at the 40hp rating these engines worked well, but the range of speeds and loads over which they could operate was quite limited. The turbocharged version unsurprisingly widened the range of speeds where maximum efficiency could be attained, and slightly better 173g/hphr peak efficiency was possible from 2400RPM all the way out to 3200RPM with fuel consumption at the rated 50hp output being just 180g/hphr.
With Volvo and BMW out of the game of small direct injection engines by the early 1990's Yanmar was left holding the bag as the producer of the smallest engines. Eventually a three cylinder 3JH displacing the same 1644cc but with a longer 3.54 inch stroke took over as the smallest direct injection engine. Rated at 35hp continious at 2900RPM this was definitely a smaller engine and was tuned to run better at lower engine speeds. Impressively high efficiency was attainable down to a 6hp load at 1600RPM where 200g/hphr was possible, but peak efficiency was a mediocre 175g/hphr from 2200 to 2700RPM. What also came with tuning the inline injection pump for lower minimum fuel consumption was poor performance up at maximum output, and these engines had a reputation for lasting best if they were not run at more than about 2700RPM. Along with numerous normally aspirated and turbocharged versions of the new two liter four cylinder 4JH this was the engine to have for quite some time.
Volkswagen also marinized their direct injection automotive engines in the 1990's, and with the more flexible electronically controlled distributor type injection pumps they took the title of smallest marine diesel engines despite their high output ratings. The same 1.9 liter normally aspirated four cylinder Volkswagen was able to run at 175g/hphr or better from 1800RPM all the way up to about 3500RPM which was way better flexibility than any of the small inline injected engines had been able to deliver. Maximum output was 58hp at 3600RPM. With the fixed injection flow rate light load performance suffered, but all the way down to 5hp at 1200 to 1400RPM 225g/hphr was possible. Peak efficiency of 165 to 170g/hphr was possible from 2100RPM up to about 3000RPM over a sizeable range of medium to heavy loads. The turbocharged 1.9TDI rated at 75hp at 3600RPM and the 2.5 liter five cylinder version with the same three and three quarter inch stroke rated at 149 or 160hp at 4000RPM were also marinized. These higher output turbocharged engines delivered a peak efficiency of 151g/hphr at about 2400 to 2800RPM, but when proped to run up to the 3600 or 4000RPM maximum engine speed they burned 175 to 180g/hphr from 2600RPM up to maximum engine speed. The five cylinder engine rated to run continuously up to 4000RPM was the truck engine in the Volkswagen model line, and did better at higher engine speeds than most other diesel engines. Under a full 136hp load at 3500RPM the 2.5TDI burned 162g/hphr.
Better performing inline injected engines certainly are possible, but they have not been offered in small sizes. The 3.3 liter four cylinder Yanmar 4TN industrial engine is a good example of this. With an output rating of 67hp at 2500RPM the 4.3 inch stroke engine is not small at all. The fact that it can deliver 156g/hphr down to 1400RPM and 175g/hphr at 2500RPM is however impressive from the perspective of a wide range of operable engine speeds. The secret of the success of the 4TN is short high pressure lines with the inline pump on the same side of the engine as the injectors.
With the electronically controlled Volkswagen diesels as the benchmark other manufacturers introduced similar products. Notable among these early 21st century offerings are the Volvo D3 and the mechanicaly injected Yanmar 4JH in it's high output 105hp trim. What is unique about the high output 4JH is that it uses a distributor type injection pump which allows really quite good efficiency over a wide range of engine speeds all the way down to less than 1200RPM. The high output 4JH does about 180g/hphr at maximum output at 3200RPM yet also delivers 185g/hphr all the way down to a six horsepower load. The Volvo D3 was aimed directly at the Volkswagen 2.5 TDI in terms of performance, but went about it differently with an electronically controlled common rail injection system. The Volvo D3 is a big engine with 2.4 liters of displacement from a 3.67 inch stroke five cylinder, but maximum power output of 217hp is also very substantial. With power output ratings of 138hp, 168hp, 197hp and 217hp all delivering the same one gallon per hour minimum fuel consumption at 1500RPM the 2.4 liter Volvo really is just masquerading as a small engine . For the 217hp high output version of the Volvo D3 performance can be considered on par with the high output 105hp Yanmar 4JH and the Volkswagens simply because it makes so much power. The Volvo D3 does not do well at all with light loads, but it can fairly efficiently put out twice as much maximum power as the 105hp Yanmar 4JH. What it comes down to is that the Volkswagen engines work fairly well under any engine speed and load combination over a wide range of engine speeds from 1000RPM all the way up to 4000RPM, where the mechanically injected 4JH can deliver even better light load efficiency along the propeller curve but is not quite as flexible for changing conditions or unusual applications. The Volvo D3 delivers impressive efficiency from 3300RPM up to 4000RPM, but as the load is reduced at lower engine speeds this early common rail engine falls flat on it's face. As the engine speed is reduced below 2700RPM the efficiency plumets even with a still very substantial 70hp load, and with a light 10hp load the Volvo D3 uses close to twice as much fuel as either the high output Yanmar 4JH or the 1.9 liter Volkswagen. Even the 165hp five cylinder Volkswagen was able to maintain 225g/hphr down to less than 15hp, where the Volvo D3 needs more like a 30 or 40hp load to attain that high of efficiency. The 217hp version of the Volvo D3 does just as well under the light 10hp load as the 138hp rated version, which again is just a huge insult to anyone looking for a small diesel engine. Probably the most interesting thing about the Volvo D3 is that the engine remains impressively efficient up at 3300 to 4000RPM with all of the output ratings from 141hp to 217hp.
For 2014 Yanmar has also jumped into the common rail chase with the two liter diesels. The new common rail injected normally aspirated 4JH is up in displacement to 2.2 liters with the same 3.54 inch stroke and is available with 43 and 54hp ratings at 3000RPM. The 76 and 104hp turbocharged common rail versions of the 4JH remain at two liters displacement and rev to 3200RPM. The most striking thing about the new normally aspirated common rail Yanmars is abysmal efficiency with worse than 195g/hphr at all speeds and loads. With this poor peak efficiency the 225g/hphr that the smallest common rail engine can do down to about six horsepower looks pretty good though. The turbocharged two liter common rail engines do better, with the 104hp version able to attain nearly 175g/hphr at maximum output. Light load efficiency remains better on the high output version as well, with 225g/hphr possible down to about seven horsepower.
Just recently quite a few engine manufacturers have been coming out with two liter common rail injected four cylinder engines. The rather similar Vetus and Bukh engines are interesting because of the quite high peak efficiencies they can attain over a wide range of engine speeds under a full load. The 1910cc Vetus has very high output ratings with 138, 168 and 187hp versions. The variable vane geometry turbocharged and intercooled Vetus VF4.140E is impressive in that it can make a large 96hp output at 2200RPM where it delivers 150g/hphr even though the stroke is rather short at 3.56 inches. Near maximum output of 133hp is available already at 3000RPM where the fuel consumption is up to only 175g/hphr. All the way up to 4000RPM at the 138hp maximum output efficiency drops off to only about 195g/hphr. Information on fuel consumption under light to medium loads does not apear to be readily available. Bukh has a very similar 168hp 2.2 liter common rail injected and variable vane geometry turbocharged four cylinder engine with a slightly longer 3.62 inch stroke. Again no information about light load performance is available, but the Bukh D170 can do 155g/hphr from 1600 to 2700RPM and fuel consumption climbs to only 175g/hphr at the 3800RPM maximum rated output.
As far as smaller engines go Bukh has had a line of two and three cylinder direct injection engines for a few years. These little inline injected 3.35 inch stroke engines are rather flexible in that they are rated to run at anywhere from 2000RPM to 4000RPM with a 3600RPM maximum continuous engine speed. The 964cc two cylinder runs at a rather poor maximum efficiency of 198g/hphr from 2400 to 2700RPM at it's 19 to 20hp continuous rating for that engine speed. At the maximum continuous output of 24hp fuel consumption is up to 209g/hphr and the fuel consumption at the maximum output of 28hp at 4000RPM is not listed. The three cylinder does considerably better with 169g/hphr from 2400 to 2700RPM at the continuous rating of 29 to 32hp for that engine speed and at the maximum continuous rating of 35hp at 3300 to 3600RPM fuel consumption climbs to only 184g/hphr at 3300RPM and 190g/hphr at 3600RPM. Again no fuel consumption information is available for maximum output of 41hp at 3600RPM and no information is available about performance under lighter loads either.
Just as the Perkins 4-236 was not really a particularly small diesel engine the smallest John Deere marine engine is not really small but is able to run fairly well under very light loads. The 4.5 liter four cylinder Deere is nearly identical in stroke to the 4-236 but has a slightly larger bore to bring the displacement up to 275 cubic inches. What is interesting about the small Deere is that it uses a computer controlled rotary distributor injection pump that delivers a fixed injection flow rate. Despite the fixed injection flow rate the 4.5 liter Deere does amazingly well over a fairly wide range of engine speeds and a very wide range of loads. Maximum continuous output from the normally aspirated version is 75hp at 2400RPM where a really very good 170g/hphr is delivered. Under a maximum continuous load fuel consumption is a stellar 160g/hphr all the way from 1500 to 2200RPM and along the propeller curve this drops off to only 170g/hphr from 1600 to 2300RPM. That is really darn good light load performance from a fixed injection flow rate and even down to a 15hp load at 1400RPM efficiency stays quite high at 180g/hphr. The 136hp turbocharged version does even better with 155g/hphr peak efficiency from 2100 to 2300RPM and the same 180g/hphr efficiency at 1400RPM with a 21hp load as well as the same 170g/hphr or better from 1600 all the way up to the 2500RPM maximum engine speed along the propeller curve. Under very light loads down to 1000RPM the 4.5l Deere continues to do amazingly well for a fixed injection flow rate engine with the normally aspirated engine delivering about 225g/hphr under a very light six horsepower load. Under a slightly heavier nine horsepower load the turbocharged version turns in better than 200g/hphr down at 1000RPM.
There is also a 6.8 liter six cylinder version of the small John Deere with the same 4.17 inch bore and 5.00 inch stroke and rated at 225hp from 2200 to 2600RPM. This larger and higher output engine is tuned somewhat differently, but it's performance is also very impressive for the fixed injection flow rate. Under a full load peak efficiency is just 165 to 170g/hphr but is available from 2500RPM all the way down to 1100RPM, a very wide range indeed. On the propeller curve the same 165 to 170g/hphr efficiency is available from 1700 to 2500RPM. Under a very light load efficiency drops off a bit more for this higher output engine, but 225g/hphr can be attained all the way down to about a 12hp load at 1000RPM.
Current versions of the 4.5 and 6.8 liter John Deere use the Stanadyne DE10 injection pump, and older versions variously used the similar Bosch VE44 computer controlled rotary distributor injection pump as well as a common rail system and an inline pump. Full electronic control of start timing allows the fixed injection flow rate to work as well as it possibly can by starting the injection event as late as possible at low engine speeds. The reason that the little Deere appears to work so amazingly well despite it's fixed injection flow rate is that it runs over a rather limited range of engine speeds. With a fixed injection flow rate a lower maximum piston speed certainly does yield better light load performance. The three and three quarter inch stroke Volkswagens run at much higher pistons speeds up at their 3600 or 4000RPM maximum speeds in marine trim and consequently don't do quite as well at running under very light loads. If the five inch stroke Deere were setup to run up to 3000RPM it would need a higher injection flow rate and would not be able to do quite as well under very light loads. Of course there is also the fact that 2500RPM will always work somewhat better for diesel engines than the 3300RPM that the three and three quarter inch stroke Volkswagens need to spin to attain the same mean piston speed.
Because IDI engines are so dirty and fuel hungry most of them have been quite small, less than about 20 to 30hp, although IDI marine engines of 40, 50 and even 70hp have also been built. Most of the IDI engines had rather short strokes in the three to three and a half inch range and ran at maximum engine speeds of about 3000 to 4000RPM. Typical maximum efficiency for IDI engines was for many years about 205 to 220g/hphr in the 2400 to 2800RPM speed range, but operational efficiencies of 230 to 300g/hphr was the best that could be done in most applications. Because the efficiency drops off so dramatically when engine loads are reduced on IDI engines getting even a 300g/hphr operational efficiency required keeping the engine load above about 40%. For most IDI marine engines minimum fuel consumption was about one third of the maximum continuous fuel consumption.
Beginning in the 1990's some improvements were made to what became known as swirl chamber IDI engines, and maximum efficiencies of 195 to 200g/hphr in the same neighborhood of 2400 to 2800RPM were attained. The newer swirl chamber IDI engines also typically work a bit better under light loads, and minimum fuel consumption of as little as one quarter of the maximum continuous fuel consumption is possible. Abysmal 300 or even 330g/hphr efficiency down at that minimum fuel consumption is however still a huge problem. A few models of IDI engines over the years have attained peak efficiencies of 190g/hphr, but better than this does not appear to be possible. Most IDI engines only worked well over very narrow ranges of engine speeds; some models would run from 2000 to 2800RPM and other models would run only between 2600 and 3200PRM. The main exception to this was the Perkins 4-108 which used a rotary distributor injection pump and could therefore run from 1500 to 4000RPM. Because the 4-108 was an IDI engine though peak efficiency and light load efficiency remained pretty dismal. The early Volkswagen 1.6D was another IDI engine with a distributor type injection pump that was marinized. Like the 4-108 the 1.6D could run between 1500 and 4000RPM, although the range of loads that could be efficiently supported was not as large because the Volkswagen used a linear distributor type injection pump with a fixed injection flow rate. The Italian company Lombardini also produces a line of two, three and four cylinder IDI engines that can run quite well from 1800 to 3200RPM with a fully mechanical unit injector system. The 18hp two cylinder Lombardini was also marketed by Fiat and Iveco. The most striking thing about the aluminum block and iron cylinder head Lombardini marine engines is their light weight. Complete with heat exchanger and transmission the 18hp two cylinder weighs 218 pounds and the 37hp four cylinder weighs 293 pounds.
Small IDI marine diesel engines have been produced by a large number of companies over the years, but most of the engines still in operation were from Yanmar, Volvo, Westerbeke (Universal) and Kubota. Some other producers of IDI marine engines have been Mercedes, Lister Peter, Ford, Sole, Deutz, Sabb, BMW and Renault. The Kubota engines were marinized by Beta, Vetus, Bukh, Sole and Nanni. Many of the Kubota IDI engines can attain peak efficiencies of 195g/hphr, and the 12hp two cylinder M2.02 from Vetus is listed as being able to do 190g/hphr from 2300 to 2700RPM with fuel consumption climbing to only 195g/hphr at the 3000RPM maximum engine speed. That 190g/hphr at 2300RPM is actually amazing for an engine with just a two and three quarter inch stroke. Other short stroke IDI engines also attain their maximum efficiency at quite low engine speeds because the restrictive openings between the pre-combustion chambers and the main combustion chambers are not as much of a problem down at lower engine speeds. The 1980's Yanmar 3GM with a 2.83 inch stroke was able to attain 190g/hphr at 1800RPM when setup as a constant speed generator set engine.
There have been some small single cylinder air cooled direct injection engines available, and some of them have been available with marine gear for use in open boats. A large producer of air cooled direct injection marine engines has been Lister Peter. In the Mediterranean air cooled diesels have been very popular for powering small fishing boats either with or without reduction gears and direct injection and IDI engines from many manufacturers have been available over the years. For many years Hatz has produced no small water cooled direct injection engines, but the 1B series of air cooled direct injection engines have continued to be available. Recently the Chinese have also come out with some single cylinder air cooled direct injection engines in the same 5 to 10hp range to compete directly with the Hatz 1B series.
The German made Hatz 1B40 is about the best performing of the small air cooled engines. The most striking thing about the performance of the 10hp Hatz is that it can attain a peak efficiency of 178g/hphr at the very low speed of 2000RPM with it's short three inch stroke. The 462cc 1B40 can make 9.0hp at 2650RPM with a fuel consumption of 190g/hphr and still has the capability to rev all the way out to 3600RPM under a load where fuel consumption climbs to 220g/hphr and power output increases only very slightly to 10.3hp. This would be a good engine for a small open fishing boat that normally operates in calm weather. Proped for full hull speed operation up at 3300RPM the engine would be only very lightly loaded with less than a two and a half horsepower load at 2000RPM for normal cruising. With this light of a load efficiency might not be all that great, but there would be a large reserve of power for net pulling or punching into wind and chop with an engine speed of around 2300 to 2800RPM. The capability of 3300RPM operation would simply mean that full power could be applied for dashing out at full hull speed with an empty boat. It would be about a 30 to 35 foot boat that the 10hp Hatz would work best in and top speed would likely be seven knots.
The China Diesel is a 1.0 to 1.5 liter single cylinder water cooled direct injection engine usually rated at about 20 to 25hp at around 1800 to 2400RPM. There is also a smaller water cooled single cylinder china diesel that is very similar to the small single cylinder Yanmar engines of the 1960's and 1970's. Like the old Yanmars the small china diesel is an IDI pre-combustion chamber engine with a regulator valve type injection pump. With a stroke of about three inches and usually run at 1500 to 2000RPM the small IDI china diesel is typically a dirty and inefficient little engine, but it can be made to run smoothly and pleasingly over a wide range of engine speeds and loads. Some of the small IDI china diesels were imported to the U.S. setup as 3600RPM generator set engines, but were very inefficient under anything but a medium heavy load and did not last well.
The real China Diesel, the big direct injection single cylinder engine, is a competent and capable power plant but being a slow turning single cylinder engine it's applicability is quite poor. What a large single cylinder diesel does best is power a generator or a pump, and the four and three quarter inch stroke is about perfect for either a 1500RPM 50Hz generator set engine or an 1800RPM 60Hz generator set engine. At 1500RPM the piston speed is just a bit on the low side, but light load operation is quite good. At 1800RPM the piston speed is better for good peak efficiency, but light load performance certainly does suffer. The China Diesel has found widespread application in 30 to 50 foot displacement speed boats throughout Asia, but most have been installed direct drive without reduction gears. Without a reduction gear a small fast spinning propeller has to be used, and this means that maximum bollard thrust is extremely low. The direct drive also means that the propulsion system works better and better up to quite high speeds which has lead to the development of long skinny boats that can go quite fast when empty.
The reduction gear transmissions for the big single cylinder engines are large and expensive and they do contribute to some loss of efficiency. When used with a reduction gear though the big China Diesel becomes a very capable propulsion system for large heavy boats. With 20 or 22hp available continuously at 1800RPM a large propeller spinning at 1000 or 1200RPM produces large amounts of thrust. Being able to idle down to 1000 or 1200RPM light load performance can also be quite good. The big China Diesel uses a metering collar type injection pump, but the low maximum engine speed and a very short high pressure line make for a reasonably wide range of operable engine speeds.
Where the direct drive system works best is in planing boats, and despite their 400pound heft the big China Diesels have found application in long skinny planing canoes that can go quite fast. With the injection system setup for high speed operation 30hp at 3000RPM is possible for speed boat style racing canoes. Another interesting application for both the large and small china diesel engines is on simplistic tractors. With four speed transmissions and power take offs these tractors are used for anything from electrical power generation to cruising down the highway and all manner of agricultural work in-between. Again the complex transmission on a single cylinder engine severely cuts into light load efficiency but these simple and relatively inexpensive tractors have been extremely popular.