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Old 01-03-2008, 05:26 PM   #1
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crazy idea- converting a gasoline engine to diesel

Has anyone ever heard of someone trying this?

I would want to start with a lawnmower engine:

Major differences/obstacles:

1. compression needs to be about 22:1 for diesel to ignite, gasoline engines are 9:1 so the head needs to be shaved a bunch or install a domed piston. Or a supercharger, Or another larger displacement engine (2X as large) feeding compressed air to a smaller engine that will run on diesel

2. with higher compression comes higher pressure on bearings and the block

3. a valve to meter diesel going to injector (to control engine speed)

4. injection pump (to pressurize the diesel or vegetable oil)- how about a power steering pump that was belt driven by the engine? These pumps will produce 1200 psi.- I have read that older diesels had injector pressures as low as 1700 psi, maybe a power washer pump- some of those will do 1700 psi.

5. injector (to spray the diesel in through the spark plug hole)- maybe a power washer nozzle.

6. something to time the injection- something like a rotary valve that would spin the same rate as the cam and could be indexed to inject the diesel at the peak of compression

Any encouragement, discouragement or ideas on how to do this on a budget?

The timing and metering issues seem like they will be the hardest to deal with.
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Old 01-03-2008, 05:32 PM   #2
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you'll need a stronger block to handle that level of compression.
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Old 01-04-2008, 09:23 AM   #3
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GM is a good example of why this is a bad idea, their engine blocks, heads, head bolts connecting rods, and nearly every part of their engine was not strong enough.
for a lown mower engine you are als going to need a heavy fly wheel, I would start out with at least 20 pounds to keep your lawn mower engine turning with the high compresion, put that on a stronger crank shaft, in a stronger block, basicy dubble what you have, either that or you could make a sleeve to reduce the engine boor by about half.
personaly, I would invest $1,200 in a hatz diesel to learn how they work, befor building your own but at that point you have a 8hp diesel that works perfectly.
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Old 01-04-2008, 04:20 PM   #4
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Agreed. for the ability for it to run for more than 5 minutes before blowing up, you'll be better off just getting a small diesel.


yea...GM does a lot of things in cars that I'd NEVER do.
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Old 01-04-2008, 05:41 PM   #5
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It's not a good idea. Parts 4-6 are the heart of a diesel, and it took Bosch many years to figure out how to do it without an air blast. It was many more years before they figured out small diesels, and direct injection in them is relatively recent. Gas engines are very simple compared to diesels.

Just buy one of the little diesels from China. Why spend so much time and money to end up where you can start? It's more interesting to do something new.
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Old 01-04-2008, 05:43 PM   #6
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Lasting 5 minutes would be a success in my book if I spent less than $50 doing it and could reuse the parts on another project

I like the challenge of doing something new and different, learning from mistakes/failures and then trying to improve what went wrong (or deciding that its too much trouble to fool with).


Thanks for pointing out that it would need a very heavy flywheel (maybe a car engine flywheel bolted/welded to a pulley)

Any ideas for a cheap rotary type valve to time the injection pulse?

How about a way to meter the diesel?
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Old 01-04-2008, 05:55 PM   #7
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I don't really need a small diesel engine- its just more of an enjoyment of tinkering and coming up with something interesting that might eventually be practical if the bugs could be worked out of it.

I've done several such cheap projects in the past (homemade trailer type backhoe and homemade band sawmill, installing points on a motorcycle with a bad CDI stator, lots of go carts when I was younger, being pulled across a pond by a truck and a long rope standing on homemade plywood waterskiis...) and so far I have gotten enough money or enjoyment out of them to justify the small investment in each project.

One of my failures was my attempt to install a woodburning stove in the yard (in a very small enclosure) and use ductwork (hot air and a cold air return) to pipe the hot air in through a garage window to heat the garage.

In short, I have lots of crazy ideas- some work, some don't (anyone recall my exhaust restrictor test? )
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Old 01-04-2008, 06:05 PM   #8
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Well, before computer controls, they always used the pump to time the pulses. The idea is pretty simple: a piston pump is synced with the motor, so the peak pressure happens at the right spot. The pump has a rotating sleeve, controlled by the governor, which allows the diesel to spill out until it reaches high enough. All diesels need speed governors: they aren't stable like gas engines.

The injectors have a sort of poppet-valve in them, so that when the pressure is high enough it lifts it off its seat to do the injection. If it didn't do that, then diesel would leak in all the time, and the pulse would be too wimpy to really atomize the diesel.

The earliest ones were air-blast: they blew the diesel in with super high-pressure air, which atomized the diesel and more precisely timed the pulse. If you can't/don't want to get a real diesel injection pump and injector, that'd be the way to go. It needs to be a very fine mist to burn. Big drops just smoke a little, but don't burn.
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Old 01-04-2008, 06:34 PM   #9
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Randy- thanks for all the info.

I believe my old diesel tractor uses the poppet style injectors, now I understand why they just can't be simple nozzles.

So how many psi would the air blast have to be? And how did they suspend the diesel in the air- in other words exactly how did the air force the diesel into the combustion chamber?
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Old 01-05-2008, 12:16 PM   #10
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I read a book on the history of diesels once, which is where I'm getting it. Did you know that diesel did the first test with gasoline? It blew the pressure recorder across the room, nearly hitting him. He switched to heavier fuels at that point.

I think the air blast was around double the cylinder pressure, but I don't remember exactly. The fuel pump and injectors were nearly the same as now, but the poppet valve was on an engine cam. As it went up it shut off the fuel and intake to an air receiver, then opened the receiver to the injector, blowing in the fuel out of the injector. It was more complicated, but they got it working pretty quickly. At the very least you'd get an air-powered motor that can generate a lot of smoke. The more modern fuel-only design is much more touchy... I don't think there's much chance of getting one working without starting with one.
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