Thread: isuzu & LPG

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.

As I posted before, good post. and as I asked before if the story could be continued.
The attached picture I pinched off another thread here. it clearly shows the pre-ignition chamber ( in this case in the piston. other models this may be in the head) and the flame fronts or carbon lines of the under pressure injected diesel as the piston moves away from the injector.

Now to keep it simple. as bush has explained it ( well) is bang on what happens in the pre-combustion chamber, but as the pre-combustion chamber runs out of oxygen the flame will hunt, as well as the diesel being pushed out of the chamber, thus my flame torch comment. Or another way maybe think of it as 2 burns or 2 stagers.
lpg fumigation would really only effect the second stage, where the hot burning gasses rip out of the pre-combustion chamber in to the relatively cold environment below ( or above)

any way that’s my attempt to explain it simply maybe others would be better at it.

side note, i spent may hours welding up Farrrt pistons where they cracked the lip of the pre-combustion chamber, strangely the NA 908 Cummings occasionally did the same but the turbos didn’t ( same pistons. 330 & 400 hp respectively)

oh if your diesel is direct injection, forget about the above you doint have the chamber.

Still going here

Just a little unrelated, but keeping with EGT's, I hade my intercooler pipe blow off this arvo, usually 17psi, so continued to drive, noted that my EGT's got to over 550C post turbo quite easily, but I'd dare say the temps pre turbo and that high as the turbo doesn't have the backpressure to create more heat as its not pumping boost

Poooouuured black smoke though

Originally Posted by clean32

oh if your diesel is direct injection, forget about the above you doint have the chamber.

All 4BD1's are direct injection. 200/300tdi's also.
They have no pre-combustion chamber, only a combustion bowl cast into the piston.

There is no "g" in Cummins and never has been.

Originally Posted by Dougal

All 4BD1's are direct injection. 200/300tdi's also.
They have no pre-combustion chamber, only a combustion bowl cast into the piston.

There is no "g" in Cummins and never has been.

LOL its called a 'Swirl-Chamber' and as it still follows the Ricardo-Comet design and rules, well at lest initially. so it changes nothing,

Thanks for the spelling lesson, very informative

Originally Posted by clean32

LOL its called a 'Swirl-Chamber' and as it still follows the Ricardo-Comet design and rules, well at lest initially. so it changes nothing,

The Ricardo Comet is a type of swirl chamber which fits in the head of some indirect injection diesels.
It is not comparable in form or function to the piston combustion chamber in a direct injection diesel.

Originally Posted by Dougal

The Ricardo Comet is a type of swirl chamber which fits in the head of some indirect injection diesels.
It is not comparable in form or function to the piston combustion chamber in a direct injection diesel.

LOL are you sure? if so prove it!

Originally Posted by clean32

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 didn't see the relevance to this thread nor to a 4BD1. However I have attached some pages copied into pdf format.

The 4BD1 is a direct injection diesel, i.e. the fuel is injected directly into the combustion chamber. The combustion chamber is in the piston crown - see pic below.

Air entering the cylinder is given a swirl by the shape/path of the inlet port. As the piston rises and approaches TDC the air is squished into the combustion chamber. Because the size of the combustion chamber is much less than the cylinder bore, the swirling action increases greatly - this is due to the principle of conservation of energy which is similar to why an ice skater spins faster when he/she pulls their arms and legs in close to their body.

The 4BD1 combustion chamber has a re-entrant bowl with a sharp lip, which improves efficiency and combustion by keeping the high swirl of the burning fuel/air mixture inside the combustion chamber as the piston descends in the early stage of the expansion stroke during which most of the fuel is burnt and heat is released. See pic and attached pdf file, 2nd page about halfway down left column - the 4BD1 combustion chamber is same as what Hino developed.

Indirect injection diesels have a pre-combustion chamber - see 2nd and 3rd pages of attached pdf file.

Originally Posted by clean32

LOL are you sure? if so prove it!

it would seem a retraction is in order... your "proof" has been posted by an independent person...