Intake temperature vs efficiency 5

[ebarba]

Hi!

Which one of the following two intake manifold conditions will lead to better efficiency, on a spark-ignited engine? Can someone say what is the ballpark efficiency increase between the best and worst condition?

A) 1.5 bar(a) pressure and 100ºC
B) 1.5 bar(a) pressure and 50ºC
Both conditions have the same mass flow.

The story: we are planning on turbocharging a spark-ignited engine burning gaseous fuel. The load is an electric generator working at constant power and speed. The reason we want to turbocharge it is efficiency as well as power increase. We just can't decide wether to install an aftercooler downstream of the compressor.

Is it ALWAYS a good idea to cool the compressor discharge?
[ul]
[li]On one hand I remember that all heat engines have better efficiency working on higher temperatures... so if the increase in mass that the compressor will deliver matches the power we want to reach, why bother in cooling?[/li]
[li]On the other, higher inlet temperatures should make the compression stroke more difficult... and we have knock lurking.[/li]
[/ul]

Thanks!

P.S. Don't know if this is really important, but the engine works great lean and we will keep it that way. Preliminary testing the selected turbocharger showed that this is OK even in the boosted operation.

 

[Panther140]

If you are talking about volumetric efficiency, then this is easy. You're going to have better luck with cooler air. If you are talking about thermal efficiency, there is a complex balance of factors that are going to play a role into the most efficient IAT for your application.

I don't believe you will have the same mass flow (Kg/Hr) of air if the temps are different, but the pressures are the same.


With higher intake temps, the air is less dense at any given pressure than it would be with lower intake temps. Therefore, you need more boost to force the same amount of air molecules into the same engine. That means that you have to apply more force to the turbocharger, which increases pumping losses and heat evacuation from the combustion chamber on exhaust stroke.

How much boost are you actually going to be running?

 

[Panther140]

You will find more efficiency as you're able to set the timing closer to where it makes Maximum Brake Torque. Combustion knock is the biggest obstacle there.

When you have higher intake temps, you have to push more air in order to get the same amount of molecules into the combustion chamber. That creates pumping losses, and you end up with a less stable fuel and air mixture. (heat decreases stability)

You would have to retard the timing more and/or run the engine with a richer AFR, because the knock limit will be stricter. That will sacrifice efficiency. I don't know which gaseous fuel you're running off of, but I would recommend Natural gas as opposed to LP for a turbocharged generator.

Will it be using EFI or a fuel mixer/carburetor? That matters for IATs




"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[MikeHalloran]

This sounds rather like an academic exercise.

Adding a turbocharger typically increases the mass air flow through the engine, if the turbo is properly sized. So for a given power level, the turbo would allow you to use an engine that's maybe a third smaller than you would need for a naturally aspirated engine at that power level.

It's also possible to size a turbo so that it adds just enough power to drive itself, and sort of loafs along, but what's the point of adding complexity that does nothing? Under that loafing condition, the turbo also doesn't make a lot of noise, and doesn't glow, so where's the fun?

With a working, properly sized turbo, and without an intercooler, you will just melt pistons for no good reason, and you will be down on power, a lot, from what you could get with the intercooler.

Example: I once worked on a boat where the engines couldn't reach their design rpm of 1600 under load, because the seawater-cooled intercoolers (CAT calls them aftercoolers) were fouled with barnacles, so the seawater flow was impeded, so the seawater coming out was too hot, and so was the air going into the engine. I don't know why the CAT techs signed off on them, except maybe because the owner's rep was such a, er, barnacle.






Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Adding an intercooler to a turbo SI engine does not seem to be standard practice for economy, at least on cars. This may be because even if it is more efficient that it is is not a cost effective way of improving economy.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[BrianPetersen]

But I know of no current turbocharged spark-ignition auto engine that does not use an intercooler.

Intercooling reduces charge temperature which reduces the tendency to knock which means not as much compromise has to be made with either the compression ratio or the ignition timing or both, and that is on the good side all around. For a given mass flow through the engine, the turbo can operate at a lower pressure ratio, and that is also a good thing.

 

[MikeHalloran]

I remember being disappointed that the Merkur XR4Ti that I wanted didn't have an intercooler, despite what one might infer from the badge.
My friend who did buy one loved it, but soon sold it, because he couldn't keep radiators in it.



Mike Halloran
Pembroke Pines, FL, USA

 

[Lou Scannon]

I remember being disappointed that the Merkur XR4Ti that I wanted didn't have an intercooler, despite what one might infer from the badge.
My friend who did buy one loved it, but soon sold it, because he couldn't keep radiators in it.

If I'm not mistaken, i=injected, both on the US spec XR4Ti 4-cyl turbocharged product, and its
Euro spec precursor, V6 normally-aspirated XR4i.
My XR4Ti now has an all brass radiator, which is about the least of the extensive mods to which it has been treated; pretty much all performance-enhancing, to be sure. I daresay it gets out of its own way a little better than average, lately.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[gruntguru]

If the engine is not knock limited it will probably have higher thermal efficiency without intercooling. The higher cycle temps will produce faster burn and lower cycle-cycle variability (especially important with lean mix - even more so if the fuel is methane based.)

The higher cycle temps will also increase turbine power allowing you to set up with lower back pressure (larger A/R housing) while running higher MAP both of which reduce pumping loss.

How do you figure the mass flow is the same for your two scenarios? Do you have a throttle downstream of the pressure measurement?

je suis charlie

 

[ebarba]

How much boost are you actually going to be running?
I don't know which gaseous fuel you're running off of
Will it be using EFI or a fuel mixer/carburetor? That matters for IATs

Boost: Between 0.3 and 0.5 bar
Fuel: wood gas
EFI or mixer: fuel mixer (actually a "tee", since the AFR here is around 1:1)

If the engine is not knock limited

unfortunately it knocks if we try to get too much out of the engine, here is a previous post on this same system about it knocking:

http://www.eng-tips.com/viewthread.cfm?qid=393108

... some of you guys will remember it.

I don't believe you will have the same mass flow (Kg/Hr) of air if the temps are different, but the pressures are the same.
When you have higher intake temps, you have to push more air in order to get the same amount of molecules into the combustion chamber. That creates pumping losses, and you end up with a less stable fuel and air mixture. (heat decreases stability)
You would have to retard the timing more and/or run the engine with a richer AFR, because the knock limit will be stricter. That will sacrifice efficiency.

I don't believe that the mass flow will be the same either, but I wanted to make the problem easier for me to chew (not a big fan of thermodynamics). I think that you have a very strong point in the above sentences.

And BrianPetersen also adds more sense to the above:

For a given mass flow through the engine, the turbo can operate at a lower pressure ratio, and that is also a good thing.

Summarizing, it makes sense to me that cooling the charge in between the compressor and the engine, will lead to less effort in putting extra mass inside the cylinders, it will be less prone to knocking and the operation would be more stable. Makes sense to you all too?

Thanks!

 

[MikeHalloran]

Reading the above, and the referenced thread, suggests that the engine is _demanding_ charge cooling. If you can pump charge cooling water from a really cold creek, do so.


Mike Halloran
Pembroke Pines, FL, USA

 

[gruntguru]

". . . cooling the charge in between the compressor and the engine, will lead to less effort in putting extra mass inside the cylinders, it will be less prone to knocking and the operation would be more stable."

No
Yes
No

je suis charlie

 

[ebarba]

". . . cooling the charge in between the compressor and the engine, will lead to less effort in putting extra mass inside the cylinders, it will be less prone to knocking and the operation would be more stable."

No
Yes
No

Can you please explain why?

 

[gruntguru]

1. OK - less effort putting air into the cylinders (assuming the intercooling is accompanied by lower boost to maintain same massflow). However the extra effort (boost) is offset by lower exhaust back-pressure (larger turbine housing) so PMEP overall is reduced.
2. Agreed
3. Cycle to cycle combustion will be more variable so I would say less stable.

je suis charlie

 

[Lou Scannon]

There are may nuances here. If spark timing can be advanced to take advantage of lower charge temperature (i.e. due to intercooling with BMEP held ocnstant), without violating a NOx constraint, should there be one, then cycle to cycle variation might be recovered to the baseline level.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[Panther140]

You're already running the engine off of wood gas, which needs to be cooled before entering the engine. Your gasses are possibly going to be hotter as a result from the increased air flow through your gassifier.

Whatever method you use to cool the wood gas could be augmented greatly to help you achieve lower IATs and condense water/impurities out of the intake supply. Using wood gas with boost is intriguing.

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[140Airpower]

"GregLocock (Automotive) Adding an intercooler to a turbo SI engine does not seem to be standard practice for economy, at least on cars. This may be because even if it is more efficient that it is is not a cost effective way of improving economy...."

"gruntguru (Mechanical). . . cooling the charge in between the compressor and the engine, will lead to less effort in putting extra mass inside the cylinders,...."

Guys, adding to the list of negative aspects of intercooling, by cooling the charge we are dumping work that the engine and turbo have expended in pressurizing and heating the charge. The benefits seem to swamp this, but for the engine that does not need the knock reduction and that is not being asked for highest specific power, it appears best efficiency should come without intercooling.

 

[gruntguru]

TheGreatDanton
"PV=nRT
Your P and V are the same
n is constant due to constant m-dot.
I think it would be safe to say that your efficiency gain is 50%."

No no no. Very unsafe.
1. You are applying the ideal gas law to what? The compression process only? That will have only a small effect on TE of the overall heat engine.
2. You cannot apply the ideal gas law to determine compressor work in the way that you have tried.
3. T must be absolute temperature - Kelvin in this case - not Celsius.

je suis charlie

 

[tbuelna]

A charge air cooler between the turbo compressor and intake manifold involves flow losses and thermal losses. So it would not likely improve an engine's thermal efficiency.

 

[Lou Scannon]

But..., it enables the engine to be re-optimised for higher thermal efficiency and power density.
Flow losses would only matter ar WOT anyway. At WOT a good intercooling system cooling effect far outweighs the flow losses.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz