James,
Farmer John asked for some input on his Chevy Sprint engine project.
I had a Sprint years ago, along with several other very-small engine autos – including a 1969 Subaru 360, Fiat with an 850 cc engine, a two-stroke-cycle DKW, and even a car with a 650 c.c. Royal Enfield motorcycle engine. The Subaru had a two-stroke-cycle gasoline engine with 25 horse. On an absolute flat highway at 55 MPH and no wind, it could get 65 MPG. My Sprint could get close to 50 MPG on a flat highway, and dropped to the high 30s on highway with steep inclines. They all relied on a formula of small engines, very light weight, and light engine-loads for good fuel economy. That is not what happens when you use such an engine for medium or high load PTO work, e.g. running a electric generator. The auto industry and./or EPA uses a measure of “miles per gallon” to reflect overall fuel efficiency capabilities of a car or truck that depend on many factors that go well beyond the engine itself. The industrial/agricultural industry uses a different measure for efficiency since the issue at hand is the engine’s ability to provided sustained medium or hard usage at a certain fuel-use rate. This figure is usually given in “horsepower hours per gallon”. These two ratings have little relationship with one another. A car engine with good fuel mileage is usually not a good candidate for hard steady use – particularly because of a short stroke to large bore ratio, compression ratio, valve lift, duration, and timing designed for highway use, et cetera. A lot of experimentation and testing has been done over the years to register fuel consumption with engines being worked via the Nebraska Testing Institute. These include engines running on kerosene, diesel, gasoline, distillate, tractor fuel, and LP (propane). One of the highest efficiency engines run on gasoline was an Allis Chalmers 33 horsepower engine – year 1962 – that provided an efficiency rating of 13 horsepower hours per gallon of gasoline. For reference, the worst gas engine tested at only 3 horsepower hours per gallon.
One of the best diesels is a John Deere tractor – actually built by Yanmar in Japan and it yielded a rating of 18.6 horsepower hours per gallon at 62 horsepower.
With engines run hard on propane – they are relatively poor performers. The best on record with Nebraska Tests is Case tractor that is rated a 9.9 horsepower hours per gallon at 71 horsepower. Propane has less energy per gallon than gasoline or diesel – there is no getting around that.
In brief – engines being worked hard tested as such in Horsepower Hours per Gallon:
Diesel – 9.2 low to 18.6 high.
Gasoline – 3.3 low to 13.1 high
Distillate – 6.7 low to 12.4 high
Kerosene – 4.8 low to 11.3 high
LP – Propane – 7.2 low to 9.9 high
Taking a look a modern cars and trucks today – they still are not much more efficient that the same built 40 years ago, when worked hard. Take a 2007 Chevy 3/4[-ton] truck with a gas engine and pull a heavy trailer, and it will do little better than a 1960s Chevy truck doing the same. However, the newer truck will do much better when driven at a light load.
One example: I recently used a 2007 Chevy truck with a 5.3 liter engine to pull a 5,000 lb. trailer and got an average of 9.2 MPG. Did the same with my 1967 Chevy truck with a 5.7 liter engine and got 9.5 MPG. Not much difference. – John in New York