# how much fuel does a diesel generator consume?April 28, 2008 4:51 AM   Subscribe

Diesel generator question: What is the typical fuel consumption rate of a 5kW, 10kW or 20kW unit at various load factors? How much does this vary (in litres per hour, or per day) between the same sizes of unit from different manufacturers?

I am doing the math to calculate the yearly operating costs for a telecommunications system to be installed in a third world nation.

The city has reliable 230V power approximately 75% of the time, which can be filtered and passed to equipment via a heavy duty voltage regulator and UPS system. That's not a problem.

The other 25% of the time, occurring randomly throughout the day there is no power whatsoever, frequently for extended periods in the middle of winter, or peak hours during summer when people are running their air conditioning. Thus the requirement for a generator.

The power budget for the whole system fits within 3/4 load factor on a 5kW generator. However we may end up with a larger unit such as 10kW to provide future expansion capability.

This Approximate Diesel Generator Fuel Consumption Chart found on Google provides basic info, but I don't know how much I can rely on it for accuracy. It covers 20kW units and above, but I don't know if a 10kW unit at 0.50 load would consume the same as a 20kW unit at 0.25 load factor...

I'm searching for a formula or chart from a major generator manufacturer saying something like "If you have a 10kW generator and operate it at Y percentage load factor for N hours, it will consume approximately Z litres of diesel per hour". The price of diesel fuel here is relatively stable, but I intend to estimate pessimistically for the price to rise 15-20% over the next 12 months.

Does anyone know how much the fuel consumption rate formula would vary between competing manufacturer's 5kW or 10kW units?
posted by thewalrus to Technology (2 answers total)

" The power budget for the whole system fits within 3/4 load factor on a 5kW generator. However we may end up with a larger unit such as 10kW to provide future expansion capability."

You've answered your question almost. If you are looking for overcapacity, that spec trumps the optimization selection of the smaller unit. Parallelling is also possible with generators, if they are phase synchronized. You could take that approach and add another 5 KW later, if you absolutely had to go with the smallest now.

My guess is that the larger unit does not require "substantially" more fuel because the load is what is going to determine most of the fuel consumption. The difference between 5KW and 10 KW is roughly 12 HP, assuming 50% generator efficiency... so that's about the engine size difference I'd expect...(just a guess).

Generac and Kohler have a fair amount of on-line info regarding sizing. I'd contact them directly and pose the question. They are in business to answer such questions.
posted by FauxScot at 9:25 AM on April 28, 2008

If you're talking about running a generator set 25% of the time, on 365 x 24 operation schedule, and sometimes for extended periods, fuel economy takes a back seat to reliability and serviceability. That's a 2190 hour a year run time average, which is a lot for small capacity generator sets, which are often air cooled diesels. Larger sets are typically water-cooled, and have much better long term life and service requirements, as a result. Another thing to be aware of is that small diesels with few cylinders have terriffic torsional crankshaft resonances, particularly at high load, due to the nature of diesel operation. So a single cylinder 5Kw unit, while meeting the electrical load spec, is going to be beating its shaft coupling to death, doing it. A 2 cylinder 10Kw air cooled unit will last a lot longer, driving the same load, but it is still going to pound its motor mounts and shaft couplings to powder a lot faster than you'd see on a gasoline unit of similar capacity. A 20Kw 4 cylinder water cooled unit will luff along for years, if you service it regularly, and you'll hardly hear its exhaust note change when the load is applied.

The bigger units also generally have better fuel filter systems, better injection control mechanisms, better facilities for air filters, oil filters, and oil changes than smaller units. That is all important, for a machine which is building over 2,000 hours of operation a year, in conditions where sandstorms are part of the regular weather pattern, and fuel quality varies.

Another consideration is that, if you only plan to have a single generator set, you probably need to back it with a battery/inverter system, capable of driving your load for an hour or so, just to give you time to take care of minor problems with the generator set, or to complete things like oil changes, if the power grid goes out while you are in the middle of maintenance. Many places with that kind of demand would have a second small generator set, at least on manual fail over switching, sitting behind a smaller inverter rig, with the inverter sized to hold the load for 10 to 15 minutes, while one or the other diesel sets comes on line. It all depends on how "mission critical" your load services are. But you have to realize that these small generator sets are nowhere near the reliability of large prime load systems.
posted by paulsc at 4:44 PM on April 28, 2008

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