Dual fuel tractors.

It depends on the octane rating of the fuels used. Octane is a measurement of flash point of a given fuel type. A gasoline/LP dual fuel set up will typically have a higher compression ratio because the octane rating of LP is about 100. Kerosene, distillate, and Tractor Fuel typically have an octane rating in the low 40's. The higher the octane, the higher the flash point. When air is compressed, it gets hot. The higher the compression ratio, the higher the heat produced in the compression stroke. Thats how a diesel engine ignites fuel. Heat created by very high compression ratios can reach 2000 degrees. Spark ignited engines don't reach compression temps like a diesel, but under load combustion chamber temps can exceede the temps required to cause low grade fuels to preignite. The ping or knock you hear is the sharp rise in cylinder pressure caused by 2 flame fronts colliding, one from a hot spot setting the fuel charge off in one area, the other from the spark plug. This is also why a diesel engine has a distinct knock. There is a sharp rise in cylinder pressure. Diesel engines need to be built stronger to handle this steep pressure rise. New diesels like the Duramax has always been and the 6.4L and newer Powerstroke use multiple injection events in each ignition event to control the rate of cylinder pressure rise. The controlled pressure rise is what makes them so quiet.

Edited by Brian Jasper co. Ia - 04 Apr 2011 at 7:52pm

chllngr528 wrote:   Reason for lower compression. Compression ignition places much more  stress on the bottom end. Sense disalite fuels such as kero and diesel have more energy then gasoline maybe the engineers were afraid that it would be to much for the rods and crank to handle?. Also, buy heating the kerosene (the obvious reason for the special manifold) you lower the flash point there by alowing the use of spark to ignite the fuel instead of compression and hence the reason to lower the compression in an effort to stop it from running on compression. If this is the correct answer I can't see the machine being very efficent nor making the power it should.

Correct for the most part. Heating the fuel allowed it to be vaporized more completely. And to answer the last part, as for being efficient or making the same power? no on both counts. Part of the fuel never does burn and it either goes out the stack or past the rings and into the oil pan. Most tractors designed to burn it had petcocks in the pan for draining down excess oil. Actualy you drained it below full and added fresh oil. What you need to realize is in the days people used low grade fuels they were cheap, sometimes close to free and because of that people were willing to put up with all the minuses. As an aside we happen to own a tractor equipped for low grade fuels and although PTO maximum HP is about the same using gasoline or our homemade distillate mixture lugging torque is greatly increased when using the lower grade fuel. We found this out when plowing on my farm. There is  a two acre "pot hole" of black gumbo dirt. The A would lug 4-16" through it in third until we ran low on fuel (5 1/2 gallons per hour) and added straight gasoline. After that we had to shift down to second gear.

Sorry, but octane number has nothing to do with flash point, which is the temperature at which a fuel can be ignited by the application of a flame to its surface as originally measured in a Pensky Martin closed cup calorimeter. Thus highly volatile fuels tend to have low flash points. Of course, I fully understand the fact that air is heated proportionately when compressed and this fact is used as the ignition principle in a diesel engine, using diesel fuel which has a high cetane number, indicating its propensity to ignite when injected into a hot compressed air charge, however, octane number indicates the opposite quality and is a measure of the ability of a fuel to withstand the heat of compression in an engine without detonating prior to the application of a spark created at the plug, and is a quality required in a fuel for spark ignition engines.  Remember that flash point is no measure of the propensity of a fuel towards pre-ignition - gasoline has a much lower flash point that kerosine, but has a higher octane number, allowing it to withstand higher compression and combustion chamber temperatures without detonating (pinging). Butch is right about a dual fuel engine developing greater torque when operated on "low grade fuel" - this is because this fuel burns more progressively when used correctly in a low compression engine and thus exerts a more constant pressure on the descending piston than is the case with gasoline. However, gasoline will produce more bhp when used in a higher compression engine of the same cubic capacity, designed designed for operation on gas. I was CEO of an oil production and refining company for many years, (and I collect dual fuel tractors) so I do know what I am talking about guys!

Edited by Tricky Dickie - 05 Apr 2011 at 11:46am

Thanks for everyones inputs

Edited by chllngr528 - 05 Apr 2011 at 4:57pm

By the way, those of you who have never experienced the exhaust perfume from a tractor running on kerosene, or tvo (tractor fuel) particularly when the tractor has a good load on the engine, have missed one of life's great pleasures. Dual-fuel tractors were the norm on this side of the pond and we UK collectors love them as a consequence! Tricky Dickie

[/QUOTE] 
As an aside we happen to own a tractor equipped for low grade fuels and although PTO maximum HP is about the same using gasoline or our homemade distillate mixture lugging torque is greatly increased when using the lower grade fuel. We found this out when plowing on my farm. There is  a two acre "pot hole" of black gumbo dirt. The A would lug 4-16" through it in third until we ran low on fuel (5 1/2 gallons per hour) and added straight gasoline. After that we had to shift down to second gear.[/QUOTE]