adrial

A professor here at Rutgers is doing research with an IR camera in the combustion chamber of some sort of Ford V8... Very interesting stuff. I'll see if I can get some pictures...

FSAEracer03

That would be great. If anyone is interested, I have some more of a diesel motor that are really interesting. I can scan them and post them if interested.

Adrial, how's your school's formula car coming along? My uprights were CNCed and are now sitting at the shop. I'm in charge of the suspension team now... which consists of me, myself and I. The only other guy involved with it graduated, got a full time job and can't do it anymore. I had to design and model up both front and rear setups within a week. So many odds and ends and specifications to play with now. Let the stress begin!

Sorry to go offtopic.

Bengt Sweden

NOx go down because temperatures go down. The combustion chamber expands rapidly. When the piston has travelled 1/9 of it's stroke (in a CR 10 engine) the combustion volume is doubled and the pressure half of what it would have been at TDC. By delaying the combustion peak pressures go down.

This is similar to exhaust gas recirculation (EGR). With already burnt and now inert gas in the combustion chamber, the combustion gets slower and thereby you do not get the peak temperatures and pressures necessary for forming of NOx. Nobody will argue for that EGR improves efficiency.
BTW high backpressure in the exhaust gives you unwanted EGR and many of the connected drawbacks.

What you normally do when running water injection is reducing the amount of extra fuel used for cooling and thereby reducing CO and HC.

Even if you did have a more complete burn you may not have maximum power efficiency of your process. You have to burn at the right time. To burn fuel when part of the stroke is already done is a waste.

Emission optimisation unfortunately seldom equals power optimisation.

Bengt

Bengt Sweden

We often blame the safety margins for the low ignition advance but part of it is due to emission regulations.
Bengt

Bengt Sweden

As we have agreed before the temperature drop that counts is the one you get at peak pressure and is not related to VE.
Actually VE can get worse if you let the water vaporize in the manifold.
What happens is that energy is transferred from the inlet air to the water. The inlet air shrinks but not as much as the volume of the water increases when it vaporizes so the water steals space and VE from the air. That is one reason why you want injectors close to the valve ( both fuel and water).

Bengt

FSAEracer03

No one said maximum efficiency, just increased efficiency. You're digressing into other subjects and not showing why you think water injection decreases efficiency. If you think the mass air count goes down due to water injection, I'd be more than glad to show you how much air is displaced by a normal water injection system versus how much density goes up. The molar count increases.

If emissions are reduced, the chemical reactions in combustion are leveling out. EGR does that by recirculating gas, yes, does that mean it's not beneficial? It's not an increase in power or volumetric efficiency, but in chemical efficiency. The combustion is more "proper" if you will. By the way, the backpressure in exhaust doesn't give any measurable gain in recirculation when the engine is designed properly (timing) and the exhaust isn't extraorinarily tiny. Exhaust gases have a far higher pressure when the valve opens than the exhaust and it leaves at sonic speed.

Naysayers will be naysayers, but water injection has proven over and over again that it DOES work. Trying to say it doesn't with mismatched arguements isn't going to refute thermodynamics or real-life testing. Sorry.

Tomas L

It's not when the valve opens that backpressure is critical, it's during the overlap period. If backpressure don't have any adverse effect on flow through the engine and VE then why does a #8 tubine increase power over a #6?

Why is it that every time you run out of arguments, you state that the laws of physics are by definition on your side? Please stick to facts, so far Bengt's arguments has been much more convincing. And yes I have read thermo and other physics courses at university level.

Tomas

mark944turbo

Any time the exhaust valve is open backpressure is critical. See Laust Pedersons thread about his cracked block, someone explains it well there. PV=nrt so if there is twice the backpressure then there is twice as much product gas in the volume left in the combustion chamber. Less heat escapes. Less room for intake to come in. This effect is obviously worse during overlap, but it is always an issue.

TurboTommy

Yes, absolutely, exhaust back pressure is always critical even if there's no valve overlap.
The piston has to do slightly more work on the exhaust stroke. The number of useless exhaust molecules in the volume between the piston and the exhaust valve will always be more at any given point right up to the time the exhaust valve closes because the pressure is always higher than a system with less back pressure. So, there will always be more exhaust left in the clearance volume. With valve overlap the whole thing just becomes worse because, obviously, extra exhaust will be forced in the clearance volume on top of that.

FSAEracer is right,
that when water evaporates (or fuel, for that matter), the molar count of the air increases (there is a net gain in air molecules available to burn fuel. When evaporation occurs there is an increase in air molecules in a given space. Yes, when the liquid water turns into gas it takes up more space, but it uses up only 20% of the gain in density in order to take up this space.
This is why the water should be evaporated quite a bit earlier before it gets to the intake ports.
If WI is installed and tuned correctly (read: less is better) a slight increase in power can be achieved even without increasing boost (providing there is already a good healthy amount of boost).
Bengt is correct, however, that timing must be optimised to achieve this. Otherwise, there will probably be a net loss.

Bengt Sweden

I just did the heat transfer calculation and yes the increase in water vapor space is less than the air shrinks. I was wrong. I am sorry that I spread a myth.

Bengt

Bengt Sweden

The reason I am not convinced about any power gains from water injection is that I have run a Volvo Turbo ( stock) at all loads and rpms with increasing amount of water with consistent power loss. At all of those load points we increased timing advance until maximum power, which was always below what we got without water.

TurboTommy, I am not sure what you mean by a "healthy amount of boost" and how that would influense. We ran stock boost levels with stock boost control system. This also means that the density gain from cooling was supported by increased flow from the turbo, and still power was down.

The focus of the study was emission control but we recorded all other basic parameters.

Bengt

Bengt Sweden

BTW, we also tested synchronised injection of the water. We had a system where we could delay injection to start at any point in the cycle just by turning a ****. There was no measurable difference at all. So starting the injection when the valve was open or closed didn't make any difference. The injectors where placed in the runners behind the fuel injectors 10cm from the valve.

Bengt

FSAEracer03

The issue between #6 and #8 is a lot more complicated than one pressure being lower than another. Saying so is shear oversimplification. Nozzle flow capabilites and flow separation have a lot to do with it... and that's a whoooole 'nother topic.

Why do I state the laws of physics are on my side in reference to water injection? Hmm... because they are? Just a guess.

FSAEracer03

Besides have a cork on the exhaust (like a cat converter... or a potato ) gains seen from lower exhaust backpressure are low. That's what I was trying to say above, so this is my an addendum. Yes, as the valve raises off it's seat and pressures equalize, gains will be seen with slightly lower pressure resistance and reduced concentration of residual gases. But I'd argue that besides extreme cases, the gains are minimal. Exhaust tuning based on primary lengths and pulse resonance can give much more effective gains... and with turbo motors, I'd really only aim to get backpressure drops for turbo spooling, wouldn't you? Also, with an NA motor, a decent backpressure can be a vital part of your power curve depending on what you're looking for: top end power or lower end torque? Many times gains in one will compromise the other.

EDIT... remind me not to type at 5:20am and attempt to be coherent on the first try!

hally

what about if we inject heavy water, increase the compression a little.. of course we would have to beef up the cat converter, on the upside knock detection could be simplified.
ok what i wanted to say is thanks for the thread
Hopefully it will end with, "to be able to run with a 993tt, remove the spark plugs, take a hammer and a centre punch..."