Spark ignited diesel

[lesharoturbo]

I have been thinking of the questions about onverting a gas engine to a diesel engine.  A few years ago I saw a report on an engine manufacturer who had experimented with a rotary engine that had a compresion ratio of 8:1 and was able to run on both gas and diesel.  It did have trouble starting on diesel but still it ran.  I thought that a spark ignited diesel would probably work better if it had a higher compression ratio say 13-14:1.  I did a google search and came across this article:

Conventional diesel engines suffer from problems of excessive weight and size and excessively high rates of pressure rise and peak pressure because of the requirement of very high-compression ratios for the purpose of ignition. They also lack multi fuel capability. The use of electrical or other precise ignition means obviates the dependence on high compression for ignition and would allow the diesel engine to operate at the more efficient and practical compression ratio of 12 to 1. To accomplish this, an innovative ignition system with an unusually high rate of energy delivery was used in conjunction with a modified diesel engine. This new design was found to provide approximately equal efficiency under most operating conditions other than high load, and to provide instant cold start at the more desirable compression ratio of between 11 to 12 to 1. However, problems of spark plug fouling by the fuel spray made this approach impractical for this specific application. Approaches for handling the problem of plug fouling have been suggested in this report.

Unfortunately, this was a government document and this is all that was declassified and allowed to be released.  The rest was "illegible" according to this site:

http://www.dtic.mil/dodsrch/docView?c=5523EBCD2CDE5FFD&dk=http%3A%2F%2Fstinet.dtic.mil%2Fstinet%2FXSLTServlet%3Fad%3DADA230243&t=y

Very interesting indeed.  If we turbo this engine and used updated injection or carb components then it just might work.  The higher compression would help to keep the plugs from fouling.

I know that the diesel cycle is different from the otto cycle in a gas engine.  From what I have read on other sites there could be a problem with higher EGT's in the gas cycle timing.  Adjusting the timing would be as easy as adjusting the distributor or crank sensor.

The possible gains could be as much as 20% increase in fuel mileage due to the higher energy in diesel and the higher ratio of air to fuel for diesel (14:1 for gas and 20:1 for diesel).

Any thoughts?

Bernie :idea: [/url]

[745 turbogreasel]

More compression=more efficiency.

Why would you want a diesel that gets poor economy under load while relying on a complicated electronic management system?

If you want one, get a tractor from the thirties, they use manifold heating to make the diesel easier to ignite if Gas isn't available.

[QuickTD]

There were many such engines in use in the first part of the century. The rumley "oil pull" was a tractor that could run on gas, kerosene, diesel or any combination of fuels using spark ignition. International also had the TD9 bulldozer in the 40's, a wonderful piece that started on gas and then switched to diesel once warmed up. Low compression made for a light (realtively) engine and spark ignition provided decent cold starting capability. Complete gasoline and diesel fuel/ignition systems were used, neither to particularly good effect, I'm told.

[TDIMeister]

The possible gains could be as much as 20% increase in fuel mileage due to the higher energy in diesel and the higher ratio of air to fuel for diesel (14:1 for gas and 20:1 for diesel).
[/url]

The difference in the energy content in a litre of Diesel fuel over the same volume of gasoline has been widely overblown and mis-stated.  Diesel fuel does have a higher heating value by volume, but not by much (about 5%).

Also, the stoichiometric air-fuel ratio is almost identical for both fuels, both being around 14.6:1 +/- 0.1 due to differences in composition by region and season.

Diesels get most of their economy advantage by a higher compression ratio, unthrottled operation, and running lean mixtures.

A spark-ignited oil engine, while maybe giving you the benefit of multiple-fuel capability, throws away most of any advantage it has, especially when running on anything but the original fuel for which it was designed.  On top of that, you add the complexity of an ignition system.

And by the way, compression ignition engines are practically by definition more "multi-fuel-capable" than spark ignited engines.  With little or no modifications, you can already run a conventional Diesel engine with Diesel fuel, Biodiesel, vegetable oil, kerosene, jet fuel, camp fuel (naphtha), and most anything else that will burn.  The only requirement is for the fuel to be self-ignitable at the temperatures reached in the cylinders at the end of compression.

[lesharoturbo]

You guys are right, there is no reason to add complexity to a diesel.  I am talking about converting a gas engine to run on diesel with a minimal to no changes.  Diesel engines are getting harder to find.

I was referring to the lean burn ability of diesel versus gas.  This helps with the increased efficiency over gas.  Also the expansion of diesel is greater than gas (about 1.5 times).  This is why the stroke of a diesel engine is usually longer than that of a gas of the same size to capture this extra expansion energy.  Gas engines tend to have a larger bore than stroke (oversquare) where diesels have longer stroke than bore (undersquare).  The increase in efficiency is a combination of all three, expansion rate, higher BTU per gallon, and lean burn ease.

If we were only worried about pure efficeincy, we would be using steam over diesel since the exopansion of water is about 4 times that of gas and 2.5 times that of diesel.

Bernie

[QuickTD]

A gasoline engine is considered a homogeneous charge because the fuel and air are thoroughly mixed prior to combustion. In order for the fuel to be ignitable in a homogeneous charge engine the fuel mixture must fall within the ignitable ratio limits, somewhere in the order of 12-15:1. The lean burn capability of the diesel engine is due to the stratified charge operation. The fuel is injected under high pressure and burns at the interface between the fuel mist and the full chamber of air, where the air fuel ratio falls within the ignitable range.

  To convert a gasoline engine to stratified charge operation you would need to add a system that could inject fuel into the combustion chamber against compression and burn pressure, essentially you would need to add a complete diesel fuel injection system to a gasoline engine. Probably be easier to just add a spark plug to a diesel...

[lesharoturbo]

Thanks for all the help guys.  I knew it wouldn't work well, but I don't know enough about diesels to know why.  Every day on this site is an education.

Bernie