Thread: isuzu & LPG

Originally Posted by Dougal

Hi Deejay.

No the volumes that detonation is being experienced with are not within those values. The LEL (lower explosive limit) and UEL (upper explosive limit) that you've posted are for igniting a cold gas with a hot spark.
This is a very different situation to heating and squeezing a gas mixture until it self-ignites.

I've got detonation at idle at 0.4% by volume (0.7% by mass) fumigation, this is under 1/5th of the LEL.

If that is the case dougal how would you attempted to explain why there is no auto detonation in a LPG powered duel fuel diesel motor?

Can someone please explain why diesel fuel exploding isn't detonation? Sure sounds like it to me. At which point does raising the peak temperature and pressure in a cylinder damage an engine? That would have to depend on the design of the engine, its cooling system, the total time overload is applied, whether oil cooling squirters are fitted under the pistons etc.

Experience on this forum shows me the 300TDi engine is a "low risk" engine for fumigation, and until someone has hammered their Isuzu engine to bits I suspect it too is suitably robust.

I am purchasing one of those EGT gauges linked to earlier in this post so will have one available to anyone who wants to try one. Obviously it won't tell you what is happening in the combustion chamber EXACTLY but it still is a useful tool for engine analysis.

Auto ignition and detonation slightly different but one can lead to another

With a diesel auto ignition is a given but it takes time for the diesel droplets to burn though. LOL think of an injector as a flame torch

With a petrol motor the fuel is either atomized or vaporized and already mixed with air. This can lead to detonation.

Diesel fed though the inlet manifold can lead to detonation.

a commonly misunderstood thing is that a flame front will move faster than a pressure wave, pressure wave being at the speed of sound and then much slower due to the density of the compressed gas. mechanical induced pressure is even though the combustion chamber where chemical or burn induced compression is uneven though the combustion chamber and that unevenness gets worse as the piston moves down increasing the volume. also as the piston moved down it exposés the liner which has not been exposed to heat or infact has been actively cooled and must rely of chemical pressure or radiated heat to burn the fuel in the areas cooled by the newly exposed liner. Or you could add LPG

I would be very surprised if the LEL and UEL held under different pressures and temperature conditions.

The auto ignition temperature for gas alone is not the issue as at idle you would have to get the IP to supply zero diesel. While there is diesel injected it will cause ignition, then it is a matter of how quick (how controlled) a burn you get for the various ratios of LPG.

pinging or pinking (as I have also seen it spelt) which is detrimental to a petrol engine I understand to be caused by advanced timing and not due to detonation. So the question I have is the issue that people are concerned about caused by pinging or detonation? If pinging; can we then reduce the problem by retarding the timing on the IP?

I know that you will lose power when running on diesel only but could this be made up for when using gas?

One for bee utey, do you alter the IP timing when you instal gas to a diesel?

It seems to me that bee utey is confusing detonation (also called pre-ignition) and diesel knock.

Detonation/pre-ignition can occur in a petrol engine because the fuel enters the cylinder mixed with the intake air. Given fuel with too low an octane rating for the compression ratio and/or a hot spot, such as carbon build up will lead to detonation/pre-ignition before the spark plug fires.

A diesel engine doesn't suffer detonation/pre-ignition because there is no fuel in the intake air during the compression stroke, until the injector opens at the correct timing advance.

There is a delay (called ignition delay) from when the injector opens to when ignition occurs. The diesel fuel is injected into turbulent air in the combustion chamber. It has to undergo a chemical change before it can ignite - heat is required for the chemical change.

When ignition occurs, the temperature in the combustion chamber rises rapidly, which speeds up the chemical change of further diesel fuel being injected.

During the ignition delay period, diesel fuel was still being injected, so there is a lot of fuel mixed with the air when ignition occurs. This leads to combustion of a large amount of fuel almost immediately, resulting in a rapid pressure rise that we know as diesel knock.

Later diesel engines address this issue by having 2 stage injectors (e.g. 300Tdi), or more recently electronic controlled, common rail injection systems have multi injection events per combustion. By these means they reduce the amount of diesel that is injected during the ignition delay period, and inject the bulk of the fuel when the temperature has risen in the combustion so the diesel knock is reduced.

It has been found that the majority of NOx is produced early in the combustion process, so the 2 stage/multi event injection reduces NOx emission.

It should also be noted that combustion only occurs in regions of the combustion chamber where there is fuel and oxygen containing air mixed at the correct stoichiometric ratio.

Not all regions will contain enough oxygen as it is being used and CO2, NOx, etc. produced. The fuel spray from the injector will not burn like a blow torch for this reason - when diesel is mixed with inert gasses like CO2 it can't burn. This is why a diesel must have an excess amount of air for complete combustion to occur - it needs oxygen.

Originally Posted by slug_burner

I would be very surprised if the LEL and UEL held under different pressures and temperature conditions.

The auto ignition temperature for gas alone is not the issue as at idle you would have to get the IP to supply zero diesel. While there is diesel injected it will cause ignition, then it is a matter of how quick (how controlled) a burn you get for the various ratios of LPG.

pinging or pinking (as I have also seen it spelt) which is detrimental to a petrol engine I understand to be caused by advanced timing and not due to detonation. So the question I have is the issue that people are concerned about caused by pinging or detonation? If pinging; can we then reduce the problem by retarding the timing on the IP?

I know that you will lose power when running on diesel only but could this be made up for when using gas?

One for bee utey, do you alter the IP timing when you instal gas to a diesel?

I don't yet go anywhere near the diesel timing as it would detract from the engine's performance on diesel only. However if an engine has been advanced and tweaked it would make gas addition problematic. I leave that to diesel mechanics but this is a steep learning curve and I will probably go there in the future.

Originally Posted by Bush65

It seems to me that bee utey is confusing detonation (also called pre-ignition) and diesel knock.

Detonation/pre-ignition can occur in a petrol engine because the fuel enters the cylinder mixed with the intake air. Given fuel with too low an octane rating for the compression ratio and/or a hot spot, such as carbon build up will lead to detonation/pre-ignition before the spark plug fires.

A diesel engine doesn't suffer detonation/pre-ignition because there is no fuel in the intake air during the compression stroke, until the injector opens at the correct timing advance.

There is a delay (called ignition delay) from when the injector opens to when ignition occurs. The diesel fuel is injected into turbulent air in the combustion chamber. It has to undergo a chemical change before it can ignite - heat is required for the chemical change.

When ignition occurs, the temperature in the combustion chamber rises rapidly, which speeds up the chemical change of further diesel fuel being injected.

During the ignition delay period, diesel fuel was still being injected, so there is a lot of fuel mixed with the air when ignition occurs. This leads to combustion of a large amount of fuel almost immediately, resulting in a rapid pressure rise that we know as diesel knock.

Later diesel engines address this issue by having 2 stage injectors (e.g. 300Tdi), or more recently electronic controlled, common rail injection systems have multi injection events per combustion. By these means they reduce the amount of diesel that is injected during the ignition delay period, and inject the bulk of the fuel when the temperature has risen in the combustion so the diesel knock is reduced.

It has been found that the majority of NOx is produced early in the combustion process, so the 2 stage/multi event injection reduces NOx emission.

It should also be noted that combustion only occurs in regions of the combustion chamber where there is fuel and oxygen containing air mixed at the correct stoichiometric ratio.

Not all regions will contain enough oxygen as it is being used and CO2, NOx, etc. produced. The fuel spray from the injector will not burn like a blow torch for this reason - when diesel is mixed with inert gasses like CO2 it can't burn. This is why a diesel must have an excess amount of air for complete combustion to occur - it needs oxygen.

Thanks, I didn't really mistake diesel knock for detonation but I wanted some more exacting explanations of the diesel combustion process. This is helping my understanding a lot.

Originally Posted by clean32

If that is the case dougal how would you attempted to explain why there is no auto detonation in a LPG powered duel fuel diesel motor?

There is.
Most of this thread is discussing how to identify and possibly avoid it.

Gas fumigation of diesels is not new, the kits which have popped up in the last few years onto the Australian market do not do anything that hasn't previously been fully explorered and sidelined.

"The Diesel Engine Reference Book" has a whole chapter on dual fuelling diesels including tables on suitable fuels. LPG is noted as having poor detonation properties compared to methane.

Gases for fumigation can be rated on a methane scale in a similar manner to rating petrols by octane.
This Swedish paper is proving interesting reading. They're suggesting a minimum methane rating (MR) of 75 for natural gas engines.
http://www.stockholm.se/Global/Frist...0och%20461.pdf

I haven't yet found the MR rating of lpg, too many false positives. But I'm still looking.

If you could inject gas into your diesel cylinders when required (i.e. second set of injectors) then detonation wouldn't be an issue.