Quote:
Originally Posted by serialk11r
If you want to use spark ignition you definitely can't run anywhere near original power because the rich area right where the injector is spraying will ignite. Otherwise you'd be running a diesel cycle on a gas engine and that could be very bad for an injector not designed to do that or the reciprocating assembly, and you might lose a lot of efficiency.
Use a diesel engine for diesel.
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However, the exceedingly high compression ratios required to reliably ignite fuel via the diesel cycle is usually followed by a commensurate structural weight penalty. The German aero-diesels of the Second World War era had poor power to weight ratios and only saw service because of their exceptional range. The specific power to weight ratios of the Orbital design would not have this weight penalty and the displacement could be juggled to meet any power requirements. Your argument goes up against a working design that is undergoing development with the United States Military.
In an application useable to the common tinkerer, one could see an advantage to use a common direct injection unit and provide an electrically heated feed line just upstream of the said injector. However, one might as well use a fluid-to-fluid heat exchanger to help in the pre-heat of the fuel once the engine is up to working temperatures. The choice of engines also expands. As an example. If I want to swap a larger 5.9 L diesel engine of the Cummins 6BT variety, I am looking at an engineering feat to support and mount a 1000 lb engine. If I chose instead to use a common Chrysler 5.9 L gasoline engine running a heated and spark ignited heavy fuel, I might not see the same torque and fuel efficiency, but I may see improved fuel efficiency over the gasoline version and not as much of a horsepower loss as one would expect as I can ignite the largely homogenous fuel mix more rapidly
allowing the engine to have a greater useable rev-range. And, the engine weighs in at "only" 500 lbs. The timing of the secondary injection event is easily coordinated with the firing of the spark minimizing the problem of damaging detonation. You must remember, at 13:1 or less compression ratio, the combustion chamber temperature is going to be considerably lower than the 285 deg C needed to ignite diesel ( as an example). Yes, turbocharging and intake heating can raise this, but you will still have irregular ignition events if you desire to run in the diesel mode. The available spark ignition avoids this.
Again, I would not use a port injector as specified in the original post. However, it is conceivable that heated tips in direct injection will be available shortly.