Generator sets, commonly known simply as generators, use a gasoline, LPG, natural gas or diesel engine to spin a generator head to produce AC current at 120 and/or 220V.
Small 3600RPM Gen Sets
Inverter Based Generators
Small gen sets in the 2000 to 7500 watt range usually run at 3600RPM and are powered by single cylinder air cooled engines. Most of course are powered by gasoline engines, but some single cylinder air cooled 3600RPM diesel gen sets are also available with about 4000 to 6500 watt output ratings. The main reason that all small generators run at 3600RPM is that they are cheaper, smaller and lighter compared to 1800RPM generators. Up at 3600RPM gasoline engines can run in late compression ignition mode where efficiency is higher and exhaust emissions are lower. For diesel engines 3600RPM means somewhat lower efficiency and higher exhaust emissions but the higher speed diesel engines and generator heads are still considerably cheaper, smaller and lighter than 1800RPM units. Gasoline engine powered 1800RPM gen sets always run in full flame front travel mode. Because full flame front travel combustion is so extremely ineficient and dirty these 1800RPM units are usually used for backup purposes and nearly always are run on cleaner burning LPG or natural gas.
Some of the 3600RPM gasoline powered gen sets can attain overall efficiency of about 280g/hphr with permanent magnet generator heads running under a 50% load. Many of the small gen sets however attain overall efficiencies in the 450 to 600g/hphr rage at half load with field winding type generator heads (for more on the difference between permanent magnets and electromagnets see Electric Motors and Generators). A very popular efficiency rating for small gen sets is about 370 to 400g/hphr, which is probably attained by using a combination of permanent magnets and field windings. Obviously the field winding type generator heads suck down a whole lot more fuel, but the permanent magnet generator sets are not doing very well under a 50% load either. Some of the gen sets with permanent magnet generator heads can do a bit better at about 250g/hphr under a full load. If the permanent magnet generator head is assumed to be 90% efficient under a full load then this works out to an engine efficiency of 225g/hphr, which is about as good as gasoline engines are ever known to do. The gen sets that can do this well are typically powered by carbureted 400 to 450cc single cylinder engines with an oversquare configuration resulting in a two and a half to three inch stroke.
Most of the 3600RPM diesel gen sets have used air or water cooled IDI diesel engines and have not attained better peak efficiency than the best of the gasoline powered gen sets. Recently some models using air cooled direct injection engines have come on the market, and they attain somewhat better peak efficiency. All of the diesel gen sets do considerably better at reduced loads, and can typically do at least as well as any gasoline gen set all the way down to less than a half load. The newer direct injection 3600RPM gen sets will of course do considerably better than the old IDI gen sets under reduced loads. The 3600RPM engine speed tends to seem a bit too fast for diesel engines, but even up at this high engine speed diesel engines have often attained 180g/hphr or better. In fact all the way up to 4300RPM diesel engines can quite easily attain higher operational efficiencies than gasoline engines.
For the gasoline engines 3600RPM is actually too slow of an engine speed, but the efficiency of the gen sets is hampered by a number of other factors as well. A rather low 8 to 8.5:1 compression ratio cuts into peak efficiency considerably, but is even more of a problem under lighter load operation where the engine then runs in full flame front travel mode. The main reason for these very low compression ratios are the un-sophisticated ignition systems on the carbureted engines. With a fixed spark timing value the only way to get the engine to run at all well over a wide range of engine loads is for the engine to run in late compression ignition mode under a heavy load and in full flame front travel mode under all lighter loads.
The newer inverter based gasoline generators can be idled down to much lower engine speeds for supporting light loads and have become extremely popular because of their pleasingly quite operation. Operating in full flame front travel mode under light loads they are however not very efficient. Light load performance is better than for the fixed speed gasoline generators, but not nearly as good as would be expected with the sophisticated electronic control systems they use. From a functionality standpoint the major problem with the inverter based generators is the delay while the engine revs up when a load is applied which does not work for many types of motor driven appliances. Some models of inverter based generators can use an external battery to boost peak output and improve load starting performance when the engine is idled down for extended run time.
The main advantage of the inverter based generators is that they can use a permanent magnet generator head for efficient high output operation without sacrificing very light load performance. As far as peak efficiency goes there are a few models that are able to do better than 300g/hphr at a 50% load, which is not far behind what the best 3600RPM gasoline generators can do. Honda set the standard for efficiency with the original EU2000i, but the Yamaha EF2000iS can do even better with 266g/hphr at 800w output. The Hyundai HY2800SEi can also deliver better than 300g/hphr under a 50% load. Many other models of inverter generators inexplicably come in at more than 400g/hphr under a 50% load. Down at a 25% load most inverter generators attain abysmal sounding efficiencies of 500 to 600g/hp hr. It is hard to believe that 600g/hphr could have given inverter based generators a reputation for being spectacularly more efficient than traditional generators, but that is what happened. Another advantage of the inverter based generators is that the gasoline engine is not restricted to 3600RPM. Under lighter loads they still spin slowly, but to make maximum output the gasoline engine can rev up closer to where gasoline engines run best. Many inverter generators rev all the way up to 4,500 or even 5,500RPM to deliver full power output. The advantages of running the gasoline engine up to higher speeds are twofold. First and most importantly gasoline engines really need to spin considerably faster than 3600RPM to attain the highest possible efficiency. The other big advantage of increasing the engine speed for maximum output is that the displacement of the engine can then be smaller, and for supporting light loads a lower displacement is nearly always a substantial advantage.
So, with all of these advantages of the inverter based generators it might be wondered why they use so much fuel. Of course the conversion losses are a significant part of this. Converting the high frequency alternating current from the generator to a direct current signal and then inverting it to a 60Hz AC signal is straightforward and can be accomplished with greater than 95% efficiency over a wide range of loads. The major problem with running the generator over a wide range of speeds is that there is then a mismatch between the output voltage of the generator and the desired constant 115 or 120V RMS output at the plug. If no voltage conversion circuitry is used then the generator is only going to run efficiently over some narrow range of engine speeds where the output voltage of the generator will match the desired output voltage. Quite conveniently heavier loads on the generator would require slightly higher engine speeds and lighter loads could be supported with slightly reduced engine speeds, this is however a very small (approximately 10%) range of engine speeds. In order for an inverter based generator to run efficiently over the very wide range of desired engine speeds from 2,000 to 5,000RPM some form of voltage conversion would have to be used. The way that most inverter generators work at this point is that they run over a narrow range of engine speeds around 4,500 to 5,500RPM to support all heavier loads, and only under radically reduced loads does the engine speed drop down to around 2,000 to 2,500RPM where a small transformer is switched into the circuit to convert the voltage up to the desired output. Another problem with the inverter generators available at this point is that the engines are oversized for the loads they support. The 99cc engine in the Honda EU2000i runs up to 5,000RPM where a gasoline engine of that displacement would be expected to quite easily make 6.5hp horsepower. That 6.5hp horsepower would be good for 4500w output with a permanent magnet generator head. The idea of running an air cooled engine at maximum output might be considered poor practice, but the reality is that this level of output is not excessive. The little 99cc Honda has a bore size of 2.2 inches, which for a water cooled four stroke would be safe up to about 20 or 22hp at 12,000RPM. Six and a half horsepower at 5,000RPM is just idling along for a 1.57 inch stroke gasoline engine of a tenth of a liter displacement. If the 99cc Honda were allowed to run up to 7,000RPM where gasoline engines begin to really make power, output could be as high as 10hp. This might be getting close to the thermal limit for an air cooled engine depending on the design of the aluminum head and the effectiveness of the cooling fan. Since maximum output is normally only required for a few seconds to start a large load the 99cc Honda might carry a maximum output rating of 6500W and a continuous rating of more like 5000W at an engine speed of 6,000RPM. This would require a load starting button which would be pushed before a heavy load was to be applied. Pushing this load starting button would bring the engine speed up to 7,000RPM for 30 seconds and as soon as the load dropped off the engine speed would be reduced back down to 6,000RPM for normal heavy load operation. The 1.57 inch stroke engine also probably should not be idled down to less than about 3,000 or 3,500RPM for light load operation since the piston speed is getting really quite a bit too slow with such a short stroke.
If a lower speed inverter based gasoline generator was desired it would ideally have a considerably longer stroke in the two to two and a half inch range. With a two and a half inch stroke the engine would much better be able to idle along under very light loads in full flame front travel mode at 2000 to 2500RPM but would still easily be able to rev to 6,000RPM or higher to support heavy loads. The longer stroke would not necessarily mean a larger displacement either. With the use of four valves per cylinder a two inch bore by two and a half inch stroke single cylinder engine displacing 129cc could easily make power up to 6,000RPM or more.
Diesel engines can attain 145g/hphr from 1000RPM up to at least 2000RPM, and 160g/hphr is common all the way up to over 3000RPM. With a 90% efficient generator head this should work out to a peak overall gen set efficiency of 160 to 180g/hphr. Since gasoline engines can attain 225g/hphr at 3600RPM, yet run so much better up at 6000RPM it would be expected that something much closer to the 145g/hphr of a diesel engine should be attainable. The 250g/hphr overall efficiency of the best gasoline powered gen sets is pretty poor, and the 400 to 600g/hphr that most gen sets operate at is really out of this world lousy performance. The reality that this 400 to 600g/hphr operational efficiency in fact represents a significant reduction in fuel consumption compared to older gen sets indicates that small gen sets have not worked very well at all.
Traditionally the best gen sets by far were the 1800RPM diesel powered units, which at some times were available in sizes as small as 6kW with direct injection engines. The 1800RPM diesel gen sets not only attained quite good peak efficiency, but were able to run down to as little 15 or 20% load without the efficiency plummeting. Permanent magnet generator heads delivered 170 to 210g/hphr peak overall efficiency, and overall efficiency remained quite good down to about one quarter load. Down at less than about a 10 or 15% load is where field winding type generator heads begin to work better than a permanent magnet generator head. Field winding type generator heads attain about a 38 to 45% efficiency over a very wide range of loads, but this is so much worse than for a permanent magnet generator head that they are really only an advantage for no load operation. Since the motor has to be twice as powerful to attain the same maximum output there ends up being almost no advantage to a field winding type generator head over a permanent magnet type generator head.
A small 1800RPM direct injection diesel gen set could be built, but at this point no small engine is available for this application. The small air cooled direct injection engines currently available have very short two and a quarter to two and three quarter inch strokes for 3600RPM operation. Even if the injection system is modified for 1800RPM operation the short stroke does not allow all that good peak efficiency. The best contender for a low speed gen set engine at this point is the Hatz 1B40 with it's slightly longer three inch stroke. With stock injection system setup the 1B40 runs best from 2,100 to 2,700RPM, and would need the static timing setting retarded a bit to run as an 1800RPM gen set engine. The three inch stroke is however still quite a bit too short for 1800RPM operation, and at 462cc the 1B40 is not that small of an engine. With almost six and a half horsepower available at 1800RPM the 1B40 would be used on a 4kW 1800RPM gen set. The ideal small 1800RPM generator set engine would be a four to five inch stroke radically under square single cylinder engine. Since the point of a small single cylinder generator would be to support small loads a small displacement and radically undersquare engine would of course work best. Again if the point is to support small loads then maximum intake air flow is not essential, and an extremely radically undersquare engine would have no trouble running efficiently at 1800RPM. A four inch stroke 1800RPM engine could easily work quite well with just a two inch bore which would yield a 200cc engine. A 200cc diesel engine might make more than three horsepower at 1800RPM, but a radically under square engine for supporting light loads would probably have a slightly lower maximum power output. With a permanent magnet generator head an output rating of 1800W continuous might be reasonable. Fuel consumption under a full load would be an eighth of a gallon per hour, and efficient operation would be possible all the way down to about one thirty-second of a gallon per hour.
This is a very small generator at just 1800W output, but it could be extremely useful for powering a small battery based electrical system. The best way to use such a small generator would be for charging of batteries, and loads would not be run directly from the generator. Ideally the output voltage of the generator would be the same as the setpoint charge voltage of the battery system, but 85 to 90% efficient battery chargers are also available. If the output of the generator were 120V 60Hz nothing would be plugged into the generator but an 1800W battery charger. The entire electrical system would then run off of a large inverter, and the generator would be used for charging batteries and augmenting large loads. Through a battery charger the 1800W inverter would be able to charge a 12V battery system at about 120A, and taper charging of a lead acid battery bank could be done quite efficiently all the way down to 20A or less.
When the generator was running loads would be supported through the battery charger and the inverter, each of which contributes to some reduction in efficiency. A 90% efficient battery charger and a 90% efficient inverter would result in an 80% total efficiency for supporting loads while the generator was running. That is a reasonable efficiency for applications where the loads are small and/or highly intermittent and the generator is used mostly just for battery charging. There are also inverter/charger products available now that will augment a small generator. The way that these inverter/chargers work is that the output of the inverter can be paralleled to the output of a generator so that loads can be run directly off of small generators. When the applied load is less than the output of the generator the extra power is used for battery charging. When the applied load is greater than the output of the generator the inverter makes up the difference by drawing power from the battery bank.
The load level of the generator can be set to any value on these inverter chargers. This is important because many generators will attain slightly higher peak efficiency at 70 to 90% of their rated continuous output as opposed to being run right at maximum continuous output. For lowest emissions and longest engine life it is always best to run engines continuously at or slightly below where peak efficiency is attained. Running an engine under such a heavy load that efficiency drops off always leads to higher emissions and necessitates more frequent lubricating oil changes. For charging batteries it never makes sense to load an engine beyond the point of peak efficiency. Some of the augmenting type inverter/chargers even have separate generator load level settings for charging and running loads. In this way battery charging can be set down at or slightly below the maximum efficiency point for longest engine life and lowest emissions but the generator can still be loaded up to or close to itís maximum continuous rating for running large loads. In this way the 1800W 200cc 1800RPM generator might only be loaded to 1500W for battery charging at 100A, but when a load was applied that exceeded 1500W the generator could then be loaded all the way up to 1800W and only above 1800W would the inverter draw from the batteries to augment output.
The augmenting type inverter/chargers have become available from a large number of manufacturers in recent years, but the older style of inverter/chargers that can back off on charging current when a load is applied but cannot augment generator output also continue to be popular. These inverter/chargers require a generator large enough to supply all loads while the generator is running. For most even rather small electrical systems this means at least a 3500W generator. Many of these non-augmenting type inverter/chargers have quite large 140 or 170A 12V battery chargers to make better use of the large generator required. Making use of 170A of charging current though requires at least a 500Ahr lead acid battery bank, and a 700Ahr battery bank would normally be considered the minimum size for such a large charging current. That is a very large battery bank for a small electrical system. The huge 170A charger would be about a 2700W load, which is a substantial load for even a 5000W generator. It is easy to see that trying to charge batteries with one of these non-augmenting inverter/chargers tends to lead to a very large capacity electrical system.
The original type of inverter/chargers from the last century could not even back off on charging current when loads were applied. What this meant was that the generator had to be sized large enough to power the applied loads plus the draw of the charger at full output. These type of battery chargers really did not work well at all because they required a 5000W or larger generator just to charge at 70 or 100A 12V, which was not very efficient. To make matters even worse these older chargers were only about 75% efficient, meaning that a 100A charging current put as much as a 2kW load on the generator. The only type of generators that worked at all with this type of inverter/charger were the 1800RPM direct injection diesel generators. A 6kW 1800RPM direct injection diesel generator had plenty of power to run a 100A charger plus a variety of large applied loads and was still able to run along fairly efficiently running just the battery charger as the charging current tapered off down to 40A 12V.