Yes, LPG-Diesel blends have been tested in CI engines with some promising benefits. The use of in-cylinder injected LPG improves smoke and NOx emissions and in some cases CO emission can be minimised, specifically in high engine load. LPG spray combined with diesel fuel in the cylinder promotes diesel atomisation, increasing the velocity of diffusion combustion, which in turn diminishes soot formation. Increasing LPG fraction in the dual fuel reduces soot and NOx emission. However, LPG has low cetane number, hence a centane enhancing additive is needed when it LPG is added to diesel fuel.

Ray ignited Diesel engines are state of the art and widely used to combust gas together with approx. 5% Diesel fuel. They can switch between 100% Diesel operation and 5% Diesel operation with 95% gas.

You can use LPG is CI engine.

The pressure of LPG is approx. 2-4 bar. This is not enough to enter to the cylinder during the compression stroke through an injector.

During the suction stroke you can add it into the intake manifold, and in the cylinder the LPG mix with diesel (and also air) during the compression.

But i guess diesel wont burn in compression stroke.

Mr. Toth is basically correct, and this parallel's the way H2 is used to boost mileage on an IC gasoline engine, too. In the intake stroke, one would expect the LPG to not just distribute an aerosol of droplets, but also to gasify a good portion of its volume (possibly all of it, at low concentrations, although the benefits of higher LPG concentration reducing soot and NOx emissions as well as CO and soot formation as Mr. Okafor mentions). However this is essentially the same thing that T. Boone Pickens is promoting as part of his "Pickens Plan" to supplement diesel fuel with methane (aka natural gas, of which his companies have a surplus) for long haul heavy duty trucks and convert urban trucks to run on pure methane (aka CNG) because of low emissions (as has already been proven by bus fleets like those in Los Angeles). However LPG and CNG have a >0 cetane number, but a huge Octane number (127) (compared to gasoline in the 89 to 99 range, usually, depending on the standard by which you measure octane) (see also http://www.engineeringtoolbox.com/alternative-fuels-d_1221.html )

That is because LPG and CNG (already being compressed) don't tend to ignite when compressed again by a CI engine, however the ignition of the diesel fuel due to compression (the cetane factor) ignites the LPG or CNG gas as well. The "energy" in the LPG or CNG (as measured by the LHV for each) is slightly more than that of diesel #2 (standard diesel fuel) by about 5% to 10%, so I would disagree that a cetane "enhancing additive" would be needed. Theoretically at least, you could either save 5 to 10% on the diesel fuel needed and still have the same power, or you could use the same amount of diesel fuel and get a boost in the power available from 5 to 10%. In fact you could even have a "switch" that allowed EITHER fuel economy or extra boost of power (going up a steep grade for instance).

Since we are talking about "theoretical" engines for the moment, remember too that the LPG could be introduced to the cylinder during the compression stroke by having it come from a separate (dual) fuel injector that provides higher pressure. On the other hand, from my observation of the carburation (a very finicky adjustment) of the Yamaha "twin jet" 100cc motorcycle back in the late 1960's, I would suspect that would be a waste of engineering, and you'd get the best bang-for-your-buck by introducing the decompression stroke to mix with the air.

Sincerely,

Stafford "Doc" Williamson

I agree to Mr Ekenechukwu C Okafor partially if we go for increasing the cetane number of LPG in diesel platform with some additives it may reduce the HC emissions,but on other hand the PM emission will try to shoot up due to rise in temperature in the cylinder