Email of the day, Calculating Fuel Consumption

A great Question from Eric Today.

Message Body:

Hi George, I have been trying to find info on how to calculate the fuel consumption for a backup generator that I would like to build, but am not having much luck. Nobody I know has a clue where to look and I have looked online quite a bit. I’m guessing that I am just not looking in the right place. It’s easy to calculate the total volume consumed, but I’m not sure how to figure the liquid fuel converted to vaporized fuel and the air it would displace in the cylinder, and keep the A/F ratio in the correct range. (I am considering propane or gasoline) Any help you can offer would be greatly appreciated. I have always been a hands on DIY person, and the more I learn, the more I realize how much I don’t know. But I enjoy learning new things. Thanks for putting your work online. I enjoy the articles. Sincerely, Eric H.

Eric, the wisdom in what you say! talk to any Gang Greenie, and you’ll soon learn the vast majority have no doubts as to what they know!  The Opposite of skeptical really is gullible, and there really are good examples of oxymoron close at hand. today’s example I give,  Huffington:Scientific.

Maybe the first question is what kind of generator should we build?

It was not long ago I saw a bone head article in a popular auto magazine about E85 delivering better mileage in an auto than petrol gasoline bought at the pump. The car… a flex fuel vehicle.  Obviously the author flunked 8th grade science, as it takes little more to find the flaw in his article.

Fuel consumption numbers are easy WHEN we calculate from end to end.  We best NOT make the same mistakes as Gang Greenies and work from one side towards the middle.

The internal combustion engine driving a generator head is a very inexpensive tool for the DIYer, there are some ways to fool yourself, but if you stick with the most basic rules can can do quite well.


No one uses volume when they want their work respected.. even in our hobby groups like here.  Use weight to measure fuels.

Postage scales that plug into USB ports are great tools to measure fuel used.

Loads on the generator MUST be pure resistive. (this takes your ability to screw up on calculating real power as you need not compensate for power factor.

You will use respected equipment to calculate power production, and those used power meters you can buy on ebay might be a good choice.

Attempt to keep your work and experiments inside the confines of  grams/KWH figures, longer runs are best, starting the measure when the engine is at operating  temp. Recording air temp, cooling water temp, exhaust gas temp and more add value to your work.

Of course there’s more.. checking valve lash, engine condition, using the appropriate lube, it can all make a difference, and should be part of your notes.

Once you have a base line, you can attempt to make more power with better intakes and exhaust, better economy, less noise and more.

This can lead to fun. Others, please consider adding your recommendations in comments.. including taking me to task..






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5 Responses to Email of the day, Calculating Fuel Consumption

  1. John Gillen says:

    When I am doing rough calculations in my head I use the following figures. These will get a person in the ballpark.

    * 1 hp = 2545 btus therefore at 25% efficiency for a gasoline engine I round it off to 10,000btu of fuel/hp ( the actuall calculation = 2445/.25 = 10180 btus)
    * Gasoline = 120,000 btus/gallon this can vary by season and supplier
    * Propane = 90,000 btus/gallon
    * Natural gas = 1000 btus/cubic foot ( spec is 800 -1200 ) In winter when demand is high gas companies are aloowed to inject air into the system to increase pressure. At high demand times you are probably closer to 800. No way to know.

    Based on the above a gallon of gas will last 0ne hour for a 12hp engine running at full load or two hours for a 6 hp engine.

    * It takes roughly 2 hp engine for each kw of generator capacity

    * Hp = A rough calculation for a four stroke low compression engine running @ 3600rpm, divide cubic inches by 2. Using this formula the calculated hp will be a little high but again I said in the ballpark. Example a 44 ci engine is roughly 22hp @3600rpm. If it runs at 1800 divide by 2 again = 11hp. Using the same formula 84 ci running @ 600 rpm = 7 hp

    I have a 2.5kw genset that I ran on gasoline for 4 days during a power outage. The average fuel consumption was .32 gallons/hour. So lets do a sanity check of the formulas I just gave. .32 gals X 120,000 btus/gal = 38,000 btus/hr.

    38,000btus/hr divided by 10,000 btus/hp = 3.8 hp/hr ( average) I said 2hp/kw which for my genset would be 5hp. The genset wasn’t running at full load all the time so average hp would be expected to be somewaht less then the calculated full load hp. I think the numbers pass the sanity check. Based on the formulas I gave I was running an AVERAGE of 80% max hp. ( 80% X 5 hp = 4 hp)

    Unless you have been monitoring your loads for a long period of time you aren’t going to know exactly what your kw usage per hour is going to be. My house “average” is around 1 kw /hr +/- a few watts, depending on summer or winter. during my 4 day run at times the genset was loaded max at other times it was lopping along.

    This was a lot more then I intended to write but I got on a roll – sorry about that. 🙂 Hope this simplies what you are trying to do.

    Good luck

  2. ron says:

    Eric, what is the goal of finding the consumption numbers? If it is calculating cost of operation, the volume consumed is sufficient. You can weigh the fuel consumed, but I have never bought fuel by the pound or gram, it has always been priced by the gallon:) I doubt the tools you measure the load with are going to be precise enough to bring the volume differences caused by temperature changes into play. The biggest issue I have seen are gross inaccuracies in measuring load. Since this is the hardest part, people often estimate based on data plates on the loads. You must have good test equipment to measure accurate loads over time. The rest is attention to detail in your measurements and basic math.

    Good luck.

  3. George B. says:

    Great comments, and it’s obvious I ‘assumed’ Eric had the desire to experiment, or maybe I wanted the question to be about that?

    Both John and Ron saw that his request could be far more basic, and perhaps it is.

    The volume VS weight thing. One of the things many of us ask after we have some idea of our usage per KWH is how does this compare? Since most figures I see on small engines like the more efficient diesels are in grams/kwh or grams/horsepower hour, using the same measure can be helpful in your comparisions.

    The scale (weight) does remove a lot of the math errors, it works with all the common fuels. But opinions are just that. If I had thought more about what I wrote, I would have said.. if you use weight, you’ll stand less of a chance of making a mistake, deceiving yourself and sharing those results with others. I guess I have seen too much of that, and it has jaded me. I will long remember one person who ‘knew’ he had a 30% difference in engine performance simply by changing the compression ratio in a small diesel, and he was pretty upset that DIYers questioned his results..

    John’s mention of using the typical fuel consumption, Ron’s excellent question…. “What is your goal?” What great additions to this post!

    Sometimes it’s just learning we DIYers want to do, experimenting with timing OR comparing cost per KWH production from light loads to full loads, and seeing the difference in fuel consumption.

    In some situations, like running a 30kw gen set with a 4kw load, your pocket book is the only scale you’ll need to know your cost per KWH is higher than need be..

  4. Quinnf says:

    If you’re interested in an estimate of fuel consumption, a rule of thumb that’s pretty accurate for many diesel engines is 18 horsepower per gallon per hour. In other words, a diesel engine producing 18 horsepower will consume about 1 gallon of diesel fuel per hour. That works out to be pretty close to what has been published for the Lister 6/1, at 1/3 gallon/hour, as well as for many marine engines I have had the pleasure to know.

    • George B. says:

      Great posts, and So valuable the calcs, if we are doing much better than these figures, the first thing we do is check for math errors.

      If we add in ‘percent load’, (How well loaded the engine is), we could add another note worthy bit to our calculations. The experience Jack Belk shared The big generator at Magic, what kind of fuel it would burn light loaded, and how he’d be out of his alloted fuel in the first days of the month. The move to the Lister 6/1 allowed him to live well, and run all the hours he wanted with fuel left over.

      That ‘percent loaded’ factor can make a big difference in cost, and here is where we can do our best to embrace reality, and reflect on how others don’t.

      Reading further is optional, do so at your own risk….

      I always like to reference the worst of all loading to know how bad it can be. The Fuel/KWH figures at ‘no load’. Fuel is burned, and no work done, and dollars down the drain. The other end, a well loaded machine at 80% load or better, we obviously spread the ‘overhead costs’ across the KWHs produced. but what costs do we overlook?

      Yes, some look on and say.. so what is new, and why do we mention it? I attempt to answer.. this is one of the biggest mistakes people make, and Salesmen and Promoters of products and ideas make good use of the fact. As I’ve said before, that term ‘whole House Generator’, it is a very popular term searched for on the web, and those who use it could be far easier prey, or should I say and easier sell for something they don’t need. Load Management, and getting the work done under the management of a watchful eye, or a program will always make more sense when fuel costs or fuel availibility are a concern. When cost is of no concern, you are likely working out of the public purse!

      I reflect on the NREL, the Salesmen there who sell the public on the idea of supporting their budget. The conversation about HCPV and MJC ‘multi Junction cells’ they steer the conversation to cell efficiency and persuade the public to measure their efforts in those meaningless and unrealistic terms. It’s one of the oldest tricks, and no doubt there was a wise man who told the very same story of artful salesmen who targeted the public purse. That wise man told the stories on the streets of Athens,…. before the powers that be silenced him with a cup of hemlock.

      If we stick with cost per KWH and compare it to other solutions, it’s hard to be fooled unless we beg others to fool us. MY ongoing example of Amonix, is it $150.000 for the bare tracker? Why use it just to hold solar PV? Wouldn’t it be the ultimate patio cover?

      AS we watch the NREL, as we watch Amonix, the fact emerges, no one cares to do the eight grade math, and validate what they do according to cost/kwh.

      That single email I got after all these months, Cogentrix will meet the PPA at Alamosa! The question.. Will you admit you might be wrong? My answer.. it’s all so simple really, it’s about the cost to make a KWH, and if the Amonix 7700 makes a decent showing, there will be another field full of them soon.

      But back to engines, and following those BTUs, somewhere right now, a diesel runs cooled by water running straight through the water jacket, near cold as ice and no effort to maintain water jacket temperature.

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