Engine/Transmission (1994 - 1998) Timing Question.

[BHaner]

Could someone explain why the cylinder head pressure increases and the EGT's decreases when pump timing is advanced?



Someone is telling me that it is impossible to change cylinder pressure just by changing when the fuel is injected. I would like to know what I am talking about.



Thanks.

 

[Reb.B]

the easiest way to describe this is using a bicycle pedal. When the pedal is half way down, ... it is easy to push it. When the pedal is at the very top and u push it, it is really hard. The sooner u inject fuel at TDC the more pressure it takes to push the piston. The higher the pedal is on the bicycle the more pressure it takes to push it. This was explained to me some time ago by IT Prof. Cullen (ie. Doctor Diesel) at CSUF.

 

[Amianthus]

Just a guess.

If I were to guess, 'cause I'm not too smart, I'd say that increasing timing will increase cylinder pressures because the ignition of the fuel air mix will happen sooner. This will allow more fuel to burn earlier than would normally happen. The more fuel that burns, the more the expansion of the gasses in the cylinder. Therefore resulting in higher cylinder pressures (remember, too much is bad).



Think of it this way. Normally, you would peak cylinder pressure at around 20 degrees ATDC (I dont' know what it is actually). And your timing is at 11. 5 BTDC. You advance the timing to 16 BTDC. That would shift your peak cylinder pressure to approx. 13. 5 ATDC. The volume of the cylinder at 20 ATDC is larger than it would be at 13. 5 ATDC, but you have the same amount of fuel burning. That means, your pressure would be higher for the amount of volume in the cylinder. Clear as mud?



The lowering of the EGT's happens because you are allowing the fuel to burn longer in the cylinder (where you want it to burn) and allowing for more complete combustion. The combustion of fuel will start earlier in the rotation cycle of the engine allowing for longer time to burn before the exhaust valve opens. One of the largest contributors to EGT's is unburned fuel. This is also why you will get lower hydro-carbons in your emissions when you advance the timing. The fuel (hydro-carbon) has burned longer and less unburned fuel is being pushed out. The longer burning cycle also results in more NOx emissions. NOx is a by-product of diesel combustion. More of it means higher efficiency, but it is also one of those pollutants that isn't very good (Acid Rain).



Of course, this is just a guess.

 

[BHaner]

Ahh i see... . Just the ammo i needed Amianthus. As for Rebsram. . that's exactly what i told him the first time (Using the bicycle and everything) aparently it was too easy for him to grasp.

 

[WDaniels]

The volume of the cylinder at 20 ATDC is larger than it would be at 13. 5 ATDC, but you have the same amount of fuel burning. That means, your pressure would be higher for the amount of volume in the cylinder.

Amiantus,

The only thing is don't understand in your reasoning is if your volume is larger for an amount of given burning fuel, your pressure would be less, wouldn't it. A larger volume of space would decrease pressure. A smaller space would increase pressure, if the given amount of fuel is constant in both volumes?????????

Or am I not understanding something?

WD

 

[WDaniels]

Amiantus,

I re-read your post a couple more times and I see where I misunderstood it. Your post is most informative.

WD

 

[Amianthus]

No problem. I could have very easily misworded what I was trying to say. I'm known for that in some circles.

 

[drees1]

Amainthus, good reply, well stated. Regarding the NOx emission. That is one of the primary emissions that is controlled in our engine. It is formed at the peak pressure and temperature momentarily during the combustion cycle. Since advancing the timing increases the peak pressure, NOx is also increased. As a result most all modern diesel emission designed engines are running more retarded than ideal for economy and power due to emissions requirements. That is why advancing our timing helps us with performanc at the sake of higher NOx.



Doug Rees