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mechman600 03-05-2010 05:37 AM

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.

JasonG 03-05-2010 07:18 AM

I say go for it.
Like franklee says, just go out there and start wrenching on something.
At the worst you blow-up an old 400.
Current R&D is working on direct injection gasoline engines, diesels fed gasoline. Maybe that's backwards.
You mentioned a wye intake. How about warm up on gas, run on diesel setup?

Bicycle Bob 03-05-2010 03:23 PM

The efficiency of heat engines is limited by the Carnot equation. Higher compression gives higher temperatures, and better efficiency.

TimG 03-06-2010 12:00 PM

Both diesel and kerosene have very low octane numbers. I think in order to run a spark-ignition without detonation you would need to lower the compression ratio down to the tractor level, reducing the efficiency down below the gasoline fueled version. You have things like exhaust valves and spark plug electrodes that get way hotter than just the temperature of compressed air and will pre-ignite and/or detonate and destroy your motor in short order.

Arragonis 03-06-2010 02:59 PM

I would put a small wager on it not working at all, and a side bet on it working slightly but not very well.

Firstly I don't think the spark will make the difference between not enough heat and enough heat. If its mixed with something that does burn then that may work but I don't think you can mix fuels so easily. Maybe a gas but then that adds all sorts of other issues with plumbing - in short dunno.

The compression in Diesels is there to generate the heat.

Plus you then have timing issues - remember diesel burns slower so you may not have enough heat generated to overcome compression unless you adjust it a lot.

Thirdly diesel engines inject fuel into the cylinders and not indirectly into the induction system like Petrol cars - no throttle. I'm not sure that thick fluid like Diesel will atomise like Petrol does using Petrol injectors. Diesel won't ruin the injectors like running petrol through a diesel system will - diesel is a lubricant whereas petrol isn't.

As for diesels having less power you have to remember that power doesn't really exist, it's just an equation - (lb/ft x rpm) / 5252

Mr Duckworth (the 'worth' bit in Cosworth) expressed it more simply as power = size of bang multiplied by the number of bangs. Diesel has more power in a fixed volume than petrol has but it burns slower so the engines can't spin as fast.

The result is much more torque but at a lower engine speed, so you get less power using the calculation above. My friend's tuned 1.9 TDI makes 180hp at but at only 4000 rpm, but the torque figure is over 300 lb/ft which is V8 petrol country.

If diesels didn't have enough power (as in strength) all those trains and trucks wouldn't work very well.

mechman600 03-06-2010 06:07 PM

All true. Very true.

Here's the deal. I have a LOT of experience with diesels, and I understand them fully. I am a commercial truck mechanic on a dealership level (Peterbilt) who spends 99% of my time at work diagnosing/repairing engines, and these engines are basically giant versions of car diesels. Truck engines got into 100% computer management back in the early 90's, long before automobile diesels did. They have EGR, diesel oxidation catalysts, diesel particulate filters, variable valve timing, common rain injection, and now selective catalytic reduction as well. And all of these have been a "joy" during their various teething problems in the last number of years (read: nightmare).

There are my credentials, if needed.
Now to my preliminatry testing.

If you pour some diesel fuel on a table and try to light in on fire, it simply will not. Nothing will happen. Weird, but true. This is why diesel fuel is a much safer fuel. However, if you spray it as a mist, it will burn like crazy. Last night I loaded my spray bottle with diesel fuel, pumped 'er up, and sprayed a mist (not even that fine of a mist) through a flame, and voila, a massive fireball with very little smoke. And keep in mind the temp of our shop last night (I'm on evening shift this week) was only about 10C/50F, so pretty cold. So, misted diesel burns very well at pretty much any temperature. It doesn't require hot compressed air to burn; it only does to autoignite. And that's [maybe] what the spark plug will do.

Post 2007 Cummins engines have an injector on the outlet of the turbocharger, aptly named the "hydrocarbon doser," to provide raw HC for the catalyst for DPF regeneration. This doser works on pressures of only 200-300 PSI. I often do flow tests on these injectors by removing it, hooking up an extension supply line and harness, and enabling a test with the diagnostic software while the engine is idling so I can measure flow in a certain period of time. What I'm getting at is that this mist is a VERY fine mist of diesel fuel that lingers in the air for five minutes or more. So if done properly, I really don't think there will be an issue with atomization and making a combustable mixture in the cylinder. I just wonder if a single spark plug will be sufficient to light the whole thing on fire.

The mist from my spray bottle will not work for testing on my motorcycle. Too much fuel comes out. I'm thinking of turning a ball point pen into an injector. Hmmmmm.....

rmay635703 03-06-2010 08:06 PM


Originally Posted by mechman600 (Post 164679)
All true. Very true.

So, misted diesel burns very well at pretty much any temperature. It doesn't require hot compressed air to burn; it only does to autoignite. And that's [maybe] what the spark plug will do.
So if done properly, I really don't think there will be an issue with atomization and making a combustable mixture in the cylinder. I just wonder if a single spark plug will be sufficient to light the whole thing on fire.

The mist from my spray bottle will not work for testing on my motorcycle. Too much fuel comes out. I'm thinking of turning a ball point pen into an injector. Hmmmmm.....

Um some very old books I have read mention spark lit 2 cycle diesels, very small motors usually.

Trouble is that type of motor was used because it was lighter, simpler and was mainly there just to be able to use cheap diesel for something.

It likely would not have any of the normal advantages of a true diesel engine and would run rather rough.

thatguitarguy 03-06-2010 08:40 PM

I've never worked on diesels. Driven them plenty, but never wrenched on them.

So you've ignited your diesel mist with an open flame - have you tried it with a spark?

comptiger5000 03-06-2010 10:47 PM

As mechman said, diesel isn't terribly flammable. At the temperatures it would reach in an engine, it would likely ignite with the spark. However, I doubt it would burn well or completely, so it wouldn't be overly efficient and probably very dirty.

mechman600 03-07-2010 01:47 AM

No, I haven't tried to light the mist with a spark yet. But I just thought of something.

The 2007-2009 Caterpillar truck engines use a system called an auxilliary regeneration device (ARD). Basically, it is a flame thrower on the outlet of the final turbocharger that shoots an actual flame into the exhaust stream, as opposed to other engines (including all diesel engines in cars) that rely on a hydrocarbon doser and oxidation catalyst. Regenerating a DPF (oxidizing the collected soot in the filter, turning it into "friendly" gasses, H20 and CO2) requires a minimum of 600F, and this device ensures that this temperature can be maintained for a length of time to keep the DPF from plugging up with soot. The temperatures actually reach more like 1100F at times during these "active" regenerations, as they are called. The ARD has a built in fuel injector, is fed bleed off boost air from the turbochargers, AND (wait for it.....) a spark plug. Yes, 2007-2009 Cat truck engines all have one spark plug, and this plug is what ignites a diesel fuel/air mixture to burn the DPF clean.

This still doesn't tell me if it will work in a very short "inside the cylinder" combustion event.

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