A Biker's Blog

with turbo-what cr to run diesel in a 250r?

if a billet crank was cnc machined, built trans gearset, direct fuel injection, with a turbo located after the expansion chamber-what kind of compression ratio/psi would be needed to get the temps/air right for diesel direct injection upon the piston sealing at the top of the exhaust port-eliminating fuel losses.

Most diesels run at least 16:1 compression ratio with many modern diesels running 20-22:1. It sounds as though you’re talking about converting a 2 stroke engine. I don’t know how that would work. First off, with the ports expossed for part of the cycle you’d likely have a hard time getting the effective cr high enough. GM 2 stroke diesels used intake ports but with conventional poppet valves. For scavenging, a Rootes type blower was used to pressurize the cylinder and force exhaust gasses out. I doubt a turbocharger would work with a conventional 2 stroke design because exhaust back pressure would likely prevent the intake charge from entering the cylinder.

All diesels are actually direct injection with the injector injecting fuel into the combustion chamber. Most pre-1970 diesels used what was called indirect injection. The injector was mounted on the side of the combustion chamber and would squirt a stream of low pressure (1400-1800 psi) fuel into the chamber. The stream being too heavy to atomize and burn immediately, it would enter a “power cell” on the opposite side which in turn directed it back into the chamber. If you’ve ever turned a faucet on full stream into a drinking glass and the watershoots right back out, you know how power cells work.

Direct injection systems work under higher pressures (old ones 2400 psi, newer ones up to twice that high) and the injector has a number of tiny holes, atomizing the diesel. Compared to the old indirect systems, the early direct injection engine were more fuel effecient and would start easier, but were also much louder with far more pronounced knocking. Later, White Motors would use the German M.A.N. system which was direct but had far less knocking. Modern diesels use multiple injections of varying time periods on each injection cycle to reduce knocking. In addition, mechanical injection pumps are noisy, expensive and require considerable horsepower to run. Electronic solenoid injectors like used on modern systems would probably be more effecient, but they are very expensive and you’d need a accurate way to fire them at the right time.

Compared to a gasoline engine, a diesel has a much heavier and stronger crankshaft, connecting rod, crankpin and wristpin diameter as well as piston. The heavier weight of the reciprocating parts is why diesels generally red line way lower than do gas engines. I also bet you’d have a hard time holding head bolts and may have to use the old garden tractor pullers trick of having external threaded rods attached to holding plates on the top and bottom of the engine to keep it from popping the head off. Holding the head gasket would also probabaly be tough and using gasoline engine parts, you might be forced to cut grooves in the cylinder block and head and use copper sealing rings instead of a conventional gasket.

I know engines fairly well, but I’d hate to take on the task of converting a gas engine to diesel. GM tried that in the 1980′s with their first 350 diesel. They took a 350 ci gasoline block and converted ir over with very poor results and gave them a reputation that took many, many years to get over. Their biggest (but not only) problem was that the block would flex too much which cause all sorts of problems.

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