Thread: isuzu & LPG

Originally Posted by Dougal

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.

But Dougal, you have spent a lot of time posting that LPG fumigation will not work, mainly you have based these statements on you BBQ experiment and your claim of detonation and subsequent motor damage.

Now you are claiming some thing totally different?

Which of your posts are correct Dougal?

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.

Good stuff, would you like to continue the story with an explanation of the pre-ignition chamber, it purpose and effects on complete combustion as well as how the flame front moves from the pre-ignition chamber etc

I think I'm starting to see the light

Originally Posted by clean32

Defiantly but then you have left the world of fumigation and entered the world of duel fuel.

At what point do we move from fumigation to dual fuel, and is the ratio determined by diesel / LPG or LPG / air. After re-reading most of this thread it seems fumigation is generally around 0.5% LPG / air or 4:1 diesel / LPG by volume. Does moving towards or slightly beyond 1:1 diesel / gas (as predicted at idle) move us effectively into the region of dual fuel.

Originally Posted by clean32

yes it can, temperature ignition is no mystery see DeeJay's post, he’s correct there are a few other variables but that’s the guts of it.

Okay, so the LPG breaks up into methane / hydrogen and we then need to treat these as separate gases, and this changes the auto ignition temperature to the lower of these (will look this up).

Originally Posted by clean32

About 1deg depending. i should say 1 deg of timeing for example
Focus on the complete burn and increased midrange rather than over all KW ( leave that to the domain of the forced induction) chasing detonation is just barking up the wrong tree.

If its a constant 1deg then that shouldn't make a lot of difference, would that 1deg remain constant or be higher at lower RPM, refering back to the situation at idle.

Even the people fitting these systems state that beyond a certain ratio of LPG that severe engine damage will occur. Considering most turbo diesels will live a relatively long life with up to 50% higher diesel rates, and that fumigation is a lot lower than this, it would appear this could only be from something like detonation (some do mention it as detonation). This is probably related to the first question.

I agree that for cleaner burning / slight running cost reduction (at least in OZ) this seems to work well, and for outright power increase, turbocharging is the only way to go. But I still would like to understand exactly how this works, if only to satisfy my own curiosity.

Not so happy with the idea of acetylene though, I know engines have run on it in the past, but after seeing what it does in spud guns, it can stay right away from my engine thanks.

My shed full of things in bits are a result of curiosity.

Lyle.

Originally Posted by clean32

[COLOR=black][FONT=Verdana]But Dougal, you have spent a lot of time posting that LPG fumigation will not work,

I have never made such a claim. Stop trolling and start reading instead.

In my experience with petrol engines running on LPG, detonation happens AFTER ignition occurs, due to the advancing flame front raising the pressure/temperature of the remaining mixed gas/air mixture to auto ignition. That is why the amount of detonation varies with load: more or less mixture detonates. Glowing carbon deposits may cause ignition independently of spark ignition and are a separate concern. They will cause backfiring through the inlet and are due to excessive carbon deposits glowing under high load. Typically experienced in oily old engines being pushed hard.

On this note I have a P76 engine running 13:1 compression and yes, detonation prevention requires careful ignition control. Water injection helps too. I had a head off yesterday to replace a broken ring so I know it is sailing close to the edge. But the piston crowns and ring lands were unmarked (at 80 000km) even though I frequently experience minor detonation.

So with diesel engines the base question is, at whan level of fumigation does auto ignition occur before diesel is injected??? Or AFTER injection/ignition? That is, a LPG detonation occuring simultaneously with diesel burning. It would appear this level is possibly around 0.5-1.0% of air. Fumigation at a lower level appears unable to detonate otherwise detonation would wreck all fumigated engines. So it all comes back to controlling the gas flow to below the detonation level and this limits the useful ratio of LPG to diesel to about 25-30%. This limits fumigation systems to markets, such as Oz, where the LPG is home grown and relatively cheap.

From my reading, the LPG addition is properly described as an ACCELERANT, which when thoroughly mixed with the air advances the speed of combustion and extends it into zones previously too cool to burn, such as close to the cylinder walls. The audible result is a rising in pitch of the diesel "knock" which is NOT DETONATION but increased burning speed. This is why diesel engines can exhibit a modest increase in fuel efficiency with LPG fumigation. That means the total fuel energy delivered to the engine is lower with fumigation. A small portion of the LPG may remain unburnt (at the cylinder walls etc.) and increase the HC emissions at light loads.

It also shows injecting the LPG into the cylinder with the diesel is a waste of time in this context as the LPG would not have enough time to diffuse throughout the air volume and aid as an accelerant. Thorough LPG/air mixing is the key to best results with fumigation.

Good post, questions and comments below.

Originally Posted by bee utey

In my experience with petrol engines running on LPG, detonation happens AFTER ignition occurs, due to the advancing flame front raising the pressure/temperature of the remaining mixed gas/air mixture to auto ignition. That is why the amount of detonation varies with load: more or less mixture detonates. Glowing carbon deposits may cause ignition independently of spark ignition and are a separate concern. They will cause backfiring through the inlet and are due to excessive carbon deposits glowing under high load. Typically experienced in oily old engines being pushed hard.

There's one big difference in how petrol and diesel engines respond to load. Petrols throttle the cylinder charge so at less than full throttle there is a lower pressure and temperature in the cylinder.
On an unthrottled diesel the pressure and temperature in the cylinder before diesel injection can be the same whether it's idling or running at full power.

There are of course differences due to charge air temperature on turbocharged diesels and thermal soak as a diesel engine running at full power will probably have a hotter block and head temp than an engine at lower load.

Originally Posted by bee utey

So with diesel engines the base question is, at whan level of fumigation does auto ignition occur before diesel is injected??? Or AFTER injection/ignition?

That is indeed the base question, another question I'd like to add is:
At what level does detonation become detectable to an observer?

IMO mild detonation below an observers detection level isn't going to cause immediate failure in a diesel because the amount of fuel and therefore energy is low. It's not on the same scale as a full load detonation on a petrol.
It is probably going to accumulate damage to pistons, rings and possibly cylinder liners. This is why I'm interested in oil analysis and compression tests.
Higher counts of iron and aluminium have been observed by others in fumigated engines.

Originally Posted by bee utey

It also shows injecting the LPG into the cylinder with the diesel is a waste of time in this context as the LPG would not have enough time to diffuse throughout the air volume and aid as an accelerant. Thorough LPG/air mixing is the key to best results with fumigation.

I beleive some 2 strokes directly inject a mix of air and fuel. Such a system with LPG could work. In cylinder turbulence is already high enough to mix squirted diesel and air, it's got to be high enough to mix gas with air too.

Originally Posted by Dougal

There's one big difference in how petrol and diesel engines respond to load. Petrols throttle the cylinder charge so at less than full throttle there is a lower pressure and temperature in the cylinder.
On an unthrottled diesel the pressure and temperature in the cylinder before diesel injection can be the same whether it's idling or running at full power.

There are of course differences due to charge air temperature on turbocharged diesels and thermal soak as a diesel engine running at full power will probably have a hotter block and head temp than an engine at lower load.

Obviously detonation in a clean spark ignition engine occurs when the peak cylinder pressure has risen to the point of compressing some of the remaining portion of fuel/air to its detonation point. The throttle will be open and cylinder filling will be high. At full load therefore the analogy has some merit. The detonation happens at cylinder pressures in excess of those from compression alone, even in a diesel.

The difference between the two engine types lies mainly in the source of ignition. The spark creates a travelling flame front from a single point. The diesel engine combustion commences across the volume from multiple droplets combusting at once. The combustion would be a function of the surface area of the droplets exposed to hot air from compression. It seems likely any detonation of the LPG at this point would be rapid and complete as the entire volume rises in pressure. Detonation without diesel combustion occuring first appears to be the primary difference between your argument and mine. Removing the diesel fuel by governor, accelerator backing off or shut down appears to stop the LPG combusting in any way so I don't think the LPG ever reaches auto ignition under normal fumigation. Without the zero accel cutoff fitted I cannot pick the difference between diesel only and LPG still flowing, even after a prolonged power usage up a steep hill.

Originally Posted by Dougal

That is indeed the base question, another question I'd like to add is:
At what level does detonation become detectable to an observer?

IMO mild detonation below an observers detection level isn't going to cause immediate failure in a diesel because the amount of fuel and therefore energy is low. It's not on the same scale as a full load detonation on a petrol.
It is probably going to accumulate damage to pistons, rings and possibly cylinder liners. This is why I'm interested in oil analysis and compression tests.
Higher counts of iron and aluminium have been observed by others in fumigated engines.

I doubt if there could be mild detonation anywhere other than at the cylinder walls where diesel combustion doesn't reach. It would indeed be minor but a possible contributor to metal count in overloaded engines.

Originally Posted by Dougal

I beleive some 2 strokes directly inject a mix of air and fuel. Such a system with LPG could work. In cylinder turbulence is already high enough to mix squirted diesel and air, it's got to be high enough to mix gas with air too.

That is of course another engine type and not currently within the scope of the argument here, does it wreck your Landy engine. At the point of adding gaseous fuel with injection the case for LPG fumigation disappears and CNG comes in as the preferred fuel because of its lower auto ignition properties.

Originally Posted by bee utey

Obviously detonation in a clean spark ignition engine occurs when the peak cylinder pressure has risen to the point of compressing some of the remaining portion of fuel/air to its detonation point. The throttle will be open and cylinder filling will be high. At full load therefore the analogy has some merit. The detonation happens at cylinder pressures in excess of those from compression alone, even in a diesel.

Detonation can happen from the heat and pressure of compression alone. It's often referred to as "dieseling" and the fuel source is usually an engine's own oil. My 4BD1T did it a few times with badly worn valve-guides when it had been working hard.

It's a bit different to the whole charge igniting on a hotspot and exploding rather than burning cleanly but still related.
However, the more advanced diesel injection gets in crank angle, the higher chance that the LPG is ignited by diesel flame rather than heat and pressure alone.