Engine/Transmission (1998.5 - 2002) Engine/power

[CARDOC77]

RUSTJC: Re. heat verses compression. Chrysler has the grid heaters to warm the air on cold days which replaces the glow plugs. Compression makes the air heat. Lower compression makes an engine harder to start no heated air. Add ether and BANG! The fuel on a diesel is direct injection and is added at the time set up by the computer. The gas engine fires the injectors sometimes in pairs or etc. fuel is laying on the back of the valves waiting to go into the cylinders. Been reading the posts and lower compression requires lots and lots of fuel and boost pressure.



Chrysler lowered compression and retarded timing in an effort to meet emissions standards. No benefits to the diesel for improved fuel economy or power. My real question is what they did to lower compression, heads or pistons? On a gas engine the leaner the fuel mixture the higher the cylinder temps which accounts for high nox emissions. More fuel lowers cylinder temps.



Just some thoughts.

 

[Adam]

OWW

My head hurts, I think. Can some one tell me for sure, because I kinda lost it somewhere around the middle of Hohn's first post .



Seriously though... thanks for the time it took to formulate the replies... I'm a better man for having read them.



-Adam

 

[RustyJC]

Originally posted by CARDOC77

RUSTJC: Re. heat verses compression. Chrysler has the grid heaters to warm the air on cold days which replaces the glow plugs. Compression makes the air heat. Lower compression makes an engine harder to start no heated air. Add ether and BANG! The fuel on a diesel is direct injection and is added at the time set up by the computer. The gas engine fires the injectors sometimes in pairs or etc. fuel is laying on the back of the valves waiting to go into the cylinders. Been reading the posts and lower compression requires lots and lots of fuel and boost pressure.

Ummm..... OK. Since this was directed to me, I'm struggling to identify what I said that's not consistent with the above.

Chrysler lowered compression and retarded timing in an effort to meet emissions standards. No benefits to the diesel for improved fuel economy or power. My real question is what they did to lower compression, heads or pistons?

If the Cummins has a flat cylinder head surface, increasing fixed clearance volume could be done by increasing head gasket thickness, increasing the volume in the piston bowl, or both.

On a gas engine the leaner the fuel mixture the higher the cylinder temps which accounts for high nox emissions. More fuel lowers cylinder temps.

A typical gasoline-fueled engine is a "rich burn" engine - it operates on the "rich" side of the stoichiometrically ideal air-fuel ratio. A diesel engine is a "lean burn" engine - it operates on the "lean" side of this point. Therefore, you have to turn your thinking 180* when comparing a gas engine to a diesel. Leaner mixtures in a gasoline engine (approaching the stoichiometric point from the left [rich] side) will behave very much like richer (more fuel) mixtures (approaching the stoichiometric point from the right [lean] side) in a diesel engine - this isn't 100% accurate because, unlike the diesel, the gasoline engine operates on the Otto (spark ignited) cycle.



Since a diesel has no throttle plate and, (being a lean burn engine), theoretically always operates with excess air, adding even more air for a given amount of fuel will result in lower EGTs. The excess air acts as a thermal reservoir absorbing some of the heat of combustion.



Rusty

 

[Smokin Joes 2500]

too much

OUCH!!:{ My cranium hurts and its filling up with useless fluid right now. I think some of you are too smart for your own good or you are good B. S. 'rs or you have absolutely NO LIFE!! All I want to know is how much compression is my motor and do we or do we not have glow plugs???? I assume not but you know how that goes. Im going to pass out now

 

[Hohn]

SJ400-- no, we have no glow plugs. The intake heater grids perform that job. in my (admittedly little) experience. glow plugs are only in motors that have a prechamber. We have direct injection.



Rusty, correct me if I am wrong, but I always thought that the term "Otto Cycle" referred only to the four-cycle engine, so therefore all four-strokes are "Otto Cycle" engines, both Gas and Diesel. They all use the same four steps: suck;squeeze;bang;and blow... ... .



Get your mind out of the gutter.....



Hohn

 

[RustyJC]

Originally posted by Hohn

Rusty, correct me if I am wrong, but I always thought that the term "Otto Cycle" referred only to the four-cycle engine, so therefore all four-strokes are "Otto Cycle" engines, both Gas and Diesel. They all use the same four steps: suck;squeeze;bang;and blow... ... .

Hohn,



The Otto Cycle



The Diesel Cycle



Rusty

 

[Hohn]

OK, Rusty, now you have me wondering. .

Why does the diesel have that cap in the pressure curve that the "Otto" doesn't?



Also, those links show adiabatic and isobaric conditions-- which we know we don't have.



For others: Adiabatic- no loss of energy (no heat transfer)

Isobaric-- constant pressure



FWIW, EnDyn in Texas is VERY close to an adiabatic engine! They are pretty much the "skunk works" of gasoline internal combustion! They were the ones that started the whole "swirl" trend way back.



They have also built a sb Chevy that ran 16:1 compression with a small cam on pump gas! It's all about mixture distribution...



Hohn

 

[RustyJC]

Originally posted by Hohn

OK, Rusty, now you have me wondering. .

Why does the diesel have that cap in the pressure curve that the "Otto" doesn't?

Hohn,



That's the critical difference between the Otto and Diesel cycles. The Otto (spark ignition, for the purpose of our discussion) engine has a marked pressure increase after initial ignition of the charge. All of the charge is already in the cylinder just waiting for ignition. After ignition, combustion proceeds at a rate controlled only by cylinder conditions.



The Diesel cycle injects fuel at a controlled rate such that combustion pressure is constant as the combustion chamber volume increases (i. e. , the piston begins to move downward) - shown as "isobaric heating" in the link's animation. All of the fuel is not present when combustion is initiated - the fuel is injected during the combustion process. This gives us a lot of potential control over the combustion process - witness the pilot/main injection schema in the new "quiet" common rail engines.

Also, those links show adiabatic and isobaric conditions-- which we know we don't have.



For others: Adiabatic- no loss of energy (no heat transfer)

Isobaric-- constant pressure

No, those links show the basic thermodynamic principles of each engine cycle. It's simpler to understand and to see the differences that way - they are theoretical, but still valid.

FWIW, EnDyn in Texas is VERY close to an adiabatic engine! They are pretty much the "skunk works" of gasoline internal combustion! They were the ones that started the whole "swirl" trend way back.



They have also built a sb Chevy that ran 16:1 compression with a small cam on pump gas! It's all about mixture distribution...

Are you talking about EnDyn in Alice, TX? If so, don't get me started... ...



Rusty

 

[Hohn]

I think this is the same EnDyn. Larry Widmer and co. .



OK, now we have stumbled upon the primary advantage that a direct injection diesel engine has over a spark motor (even direct injected): the ability to "steer" combustion in terms of time/pressure curves.

If we had a slower burning fuel, we could inject a pilot charge at TDC, then inject just enough fuel to keep the fire burning until the crankshaft had rotated enough to where leverage was more optimal, then have the main injection event. The only problem I see with this is getting all that fuel burned in time to avoid excessive EGT. You would be injecting fuel later when cyl temp is lower, so the charge burns slower.



Word has it that the Cummins engineers could "dial-in" the noise level of the new common rail motor at any point between dead quiet and 2002 clatter. The combination of common rail injection with pilot injection is a great 1-2 punch.



So when is Piers gonna come out with the Common Rail retrofit for the ETH owners? That thing could have insane fueling levels!!!



hohn

 

[RustyJC]

Originally posted by Hohn

If we had a slower burning fuel, we could inject a pilot charge at TDC, then inject just enough fuel to keep the fire burning until the crankshaft had rotated enough to where leverage was more optimal, then have the main injection event. The only problem I see with this is getting all that fuel burned in time to avoid excessive EGT. You would be injecting fuel later when cyl temp is lower, so the charge burns slower.

There's something else to factor in besides crankshaft/connecting rod geometry. We also have to consider the expansion ratio as being critical to extracting as much power as possible from the combustion process. By having more combustion take place later in the power stroke, the average expansion ratio would be less, and we would dump more unrecovered heat and pressure energy into the exhaust.



That's not necessarily a bad thing if we can recover and make good use of this energy with the turbocharger. If we can't use all of it, we can do what some marine diesels do - install a turbine rotor in the exhaust downstream of the turbocharger, capture the remaining energy and (because this turbine rotor is geared to the crankshaft) return it to the crankshaft where it can perform useful work. Turbocompounding, as this is called, improves BHP output and fuel efficiency.



Rusty

 

[Hohn]

Yes, this is what i am talking about. Since static compression ratio= expansion ratio, i would assume you are talking about the expansion ratio AFTER the injection event (i. e. , how much piston travel remains after initial ignition)



One could argue that this is another case for higher static compression: it leaves more expansion ratio left after the injection event. Or, it requires less timing advance, since the charge not only burns faster (higher temp with higher compression) but has "expansion ratio" with which to extract energy from the charge.



This is where higher efficiency comes from, if i remember my Thermodynamics correctly. IIRC, Thermal efficiency is calculated by the difference between peak temperatures. Meaning, the hotter the temp at TDC, and the cooler the exhaust, the more efficient the burn (assuming equal fueling).



Since the ideal gas law gives us a (kind of) linear correlation between temperature and pressure, we can now say that efficiency is a product of pressure differentials. Meaning, the higher the peak pressure, and the lower the pressure at BDC, then the more energy we have converted to work.



That's one reason that we can say that high EGT is an indicator of inefficiency. Meaning that a given level of fueling could be more efficiently burned in a different engine (one under different circumstances like more boost). Brake Specific Fuel Consumption (BSFC) goes way up, meaning efficiency is way down. High egt means that stored chemical energy is being released too late to be converted into mechanical energy, and it's converted to heat instead. For the none techies, BSFC is the amount of fuel required to make 1HP for one hour-- it's a good measure of efficiency that can compare apples to apples when comparing way different fuels or engines.



This is part of the beautiful marriage of the diesel and the turbocharger. Now, when we over fuel, the hotter EGT isn't all gone to waste-- we can recover a lot of that energy by converting heat to boost, which in turn, lowers EGT and shows us that we are recovering efficiency.



I love my CTD more every day. .



EDIT- fixed typos

 

[RustyJC]

Originally posted by Hohn

Since static compression ratio= expansion ratio... .

Yes, basically, unless it's a Miller cycle engine. In that case, Re > Rc.

This is part of the beautiful marriage of the diesel and the turbocharger. Now, when we over fuel, the hotter EGT isn't all gone to waste-- we can recover a lot of that energy by converting heat to boost, which in turn, lowers EGT and shows us that we are recovering efficiency.

Another way of looking at this is that the turbocharger turbine is merely a continuation of the expansion process in the engine, and the turbocharger compressor is just the front end of the compression process in the engine. If you carry this line of reasoning far enough, you wind up with a gas turbine - all the engine does is produce high pressure hot gas to drive the turbine with enough power left over (after the turbine drives its compressor) to harness to an external load.



An example of this is one of our 2-cycle spark ignited natural gas fueled integral engine/compressors. Being a 2-cycle engine, scavenging air was provided by a centrifugal blower gear driven at 20x engine speed - later versions of this engine had a turbocharger in series with the blower (i. e. , the compressed air from the turbo went into the blower inlet). When we eliminated the blower and made this a "pure turbocharged" engine, we had to raise the exhaust ports to get enough exhaust energy to drive the larger turbocharger. Normally, this would lower the expansion ratio and make the engine less efficient. In this case, however, the brake specific fuel consumption and the BHP output both improved dramatically - the loss in efficiency caused by the higher exhaust ports was much less than the power that was being consumed by the gear driven blower.



Rusty

 

[me4osu]

Originally posted by RustyJC



Are you talking about EnDyn in Alice, TX? If so, don't get me started... ...



Rusty

Hee Hee. Cooper guys sure get testy when you say EnDyn around them.

 

[RustyJC]

Originally posted by me4osu

Cooper guys sure get testy when you say EnDyn around them.

There are valid reasons. I outlined them to you in a PM. See if you don't agree.

 

[me4osu]

I PM'd you a response to your PM.