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Old 03-05-2010, 05:37 AM   #1 (permalink)
mechman600
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Spark ignition Diesel engine, AKA your car

I was thinking the other day: back in the 40's and 50's, many farm tractors could be run on gasoline, petroleum distillate, or kerosene, interchangably. Running on Kerosene was acheived by heating the intake manifold with a nearby exhaust manifold, heating the kerosone fuel before it entered the carburetor, and a slight ignition timing change.

Kerosene is sort of between gasoline and diesel fuel in a way, as their flash point (the lowest temperature at which a volitile liquid can vaporize to form an ignitable mixture in air) is almost identical at ~120-150F, depending on many things of course. As a comparison, the flash point of gasoline is -40F.

I am assuming that using Kerosene in an ICE works well with a carburetor with a fairly constant air/fuel ratio, as these aforementioned farm tractors used a carburetor. However, diesel fuel in an ICE does not like a constant air/fuel ratio. Restricting intake air flow effectively lowers compression pressure, resulting in too low of compression temperature at TDC for diesel to autoignite. Besides, the entire point of burning diesel fuel for efficiency sake (besides that fact that diesel fuel contains more energy per volume than gasoline) is to eliminate the throttle valve and its subsequent pumping losses.

In a diesel engine, intake air is compressed at a ratio of between 14:1 to 23:1, depending on the engine, to get a compression temperature of above 600F at TDC so that when finely atomized diesel fuel is injected, it starts burning, as diesel fuel's autoignition temperature is ~410F, depending of course on its cetane level. Now, a 10:1 compression gasoline engine will probably reach a compression temperature of ~300F if we use simple math, well above the flash point but below the autoignition temperature of diesel fuel. The ultimite question is: what will happen at this point if a gasoline style spark is introduced at this point? AKA...will it run?

The heating of kerosene was necessary in the farm tractors because of low compression ratios of 6:1 to 7:1. Those ratios would probably only yield compression temperatures of 200F (again, using ridiculously simple math) - sort of borderline with the flash point of kerosene (especially on a cold day), hence the extra heat required. Something tells me that a 10:1 engine may work, unless of course ambient temperatures fall too low. But that's what grid heaters are for.

My first guinea pig will be a 1978 Honda Hawk 400 twin motorcycle. If I break it, who cares. I will remove the carbs and fashion some sort of 1-into-2 intake manifold. I will fill my pneumatic spray bottle with diesel fuel, pump the snot out of it and screw the nozzle tight for a fine mist. Then I will hit the starter and start dosing the intake with fuel. Who knows what will happen....

If guinea pig #1 works, it will open up a new door of possibilities. I will then transfer my experiment to guinea pig #2, an '06 Matrix. I will disconnect both O2 sensors, MAF sensor, MAP sensor (if it has one - I'm not sure), and see if the ECU is smart enough to run on only the two inputs of engine position/speed and throttle position. If yes, I will remove the throttle butterly, keeping the actuator and throttle shaft intact so the ECU doesn't think that it's missing (fly by wire throttle). The next bit is completely dependent on whether gasoline fuel injectors do well with diesel fuel going through them, because the final step of this ridiculous scheme is to drain the gasoline from the tank and replace it with diesel fuel. The ECU will then use engine speed/timing and accelerator pedal position to vary injection.

Now for power estimates. Considering this would end up being a naturally aspirated diesel engine, power would definitely be low. First off, I know compression ratio affects BFSC in a gasoline fuelled engine. However, in a diesel engine, it really has little effect. Think of a "giant" 20:1 compression as a spring. The bigger the spring, the harder it is to compress, but the harder it will spring back - sort of self-cancelling. In a diesel engine, whether the compression is 20:1 or 14:1 has no effect on efficiency, as long as the fuel is burning completely. Pinching the wastegate line on a turbodiesel to raise the boost pressure and peak compression pressure does nothing for power unless fueling modifications are also made. Therefore, I don't believe a 10:1 diesel engine will be any less efficient than a 20:1 one, providing that all the fuel is burned completely.

So, for a simple example, let's take the 1978 VW Rabbit gasoline vs. diesel. Both engines were nearly identical in design, 1.5L diplacement and the same 5000 RPM or so redline. The gasoline version had 71 HP and the diesel had 48 HP. Again, using ridiculously simple math, my '06 Matrix has 126 HP, so a NA diesel version would produce 85 HP.

Ok, I realize that this experiment will not work. If it would work, there would be a million internet posts with instructions on how to do it. But, could it be that something has been overlooked? I give it a 5% chance.

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