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Fuel Consumption Hit and Miss

David Cave

Subscriber
Last Subscription Date
07/19/2019
I recently heard a rule of thumb that fuel consumption for a hit and miss engine is : 1 gallon per hour per horsepower. Have you heard this rule or verified it. I will be running one of my engines loaded for several hours and was wondering. My first short run looked like 1/2 gallon per hour per horsepower.
 

Andrew Mackey

Moderator
Last Subscription Date
05/14/2017
The fuel usage on a hit and miss engine, depends on the load. Just idling (no load), they are very fuel efficient. I had a 2 HP Stover KA that would run about 4 hours on a pint of gas, no load. A 5 HP Hercules would run about 8 hours on about a gallon and a half. Put a load on, and it's a different story. All in all though, I think your gallon/Hr per HP as you state it, is a bit high - even for a throttled engine of the time. For todays high speed engines, that might be a close number, but not the old stuff.
Hit and miss engines made their rated HP at the point where they hit at every compression stroke. That would be like a throttled engine running wide open. For both types, that indeed is a heavy load. Most H&M engines were under rated, due to the hit and miss nature. They actually made more power than the data plate read. The lighter the load, the more fuel you saved on a hit and miss. Fuel consumption was different too, due to the efficiency of the mixers the engine used. Earlier engines used more gas due to the fact that the fuel was 'mixed' with the air. As the manufacturers became better in their making of engines, they also made better means of vaporizing the fuel, which raised the HP and lowered the use of fuel.

What size and make is your engine? What are you driving with it? How constant is the load? it all ads up!
Andrew
 

Kirk Taylor

Registered
I recently heard a rule of thumb that fuel consumption for a hit and miss engine is : 1 gallon per hour per horsepower. Have you heard this rule or verified it. I will be running one of my engines loaded for several hours and was wondering. My first short run looked like 1/2 gallon per hour per horsepower.
I believe that rule is, "one pint per horsepower-hour." I also believe that an engine manufacturer used that figure in their advertising or operator's manual. Hopefully someone will reference it here.
If I did the numbers correctly, it works out to something like 17% efficiency.
 

David Cave

Subscriber
Last Subscription Date
07/19/2019
what little experience I have would indicate less than a half gallon so maybe a pint is correct.
I have a fairly accurate belt driven dyno that I'm thinking of using to test the idea. I'll run a 3hp engine on a 2 hp dyno load and a 1 1/2 hp engine at 1 hp load and see what happens. In September when it cools off.:eek:
 

John Hamilton

Subscriber
Last Subscription Date
05/25/2019
My 6 hp IHC M would run from 8 to noon on a tank full. Run out of gas so I could eat lunch then run another 4 hours in the afternoon on a refill. It was under load (fodder cutter) about 3/4 of the time.
 

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