I think of it this way on a turbo charged diesel. Boost is from the energy that is generated from the fuel burned. So more fuel is needed to generate more boost. So from a efficiency standpoint you would want to boost low as possible in a steady state condition. And to increase the turbocharger's efficiency and tap it's wasted energy by using compounding like on the Wright R-3350 used on the Lockheed Super Constellation.
Even better if we had a fuel flow meter (actual amount of fuel used for combustion) and from there determine the optimal economy and power.

MikeyB

 

Of course, air mass is what really counts for cylinder-filling purposes, and higher boost generally equates with higher charge-air temps and the accompanying loss of efficiency.

Some have ascertained that no net loss in required crankshaft power is realized with a higher mass of intake air - that essentially it releases the energy to compress it like a spring. However, I respect the source...

Increasing an engine's compression ratio raises it's efficiency, and since a diesel isn't throttled - there's a case to be made that raising boost w/o additional fueling increases the engine's effective compression ratio and thus it's efficiency (read fuel economy and power production).

Agreed on the conventional wisdom, Mike!
However, I'm approaching from a more efficient thermal energy management point of view - specifically, regenerating a portion of the engine's waste heat that would otherwise be lost.

 

Hmmm, think the Pennsylvania railroad experimented with steam powered turbine locomotives (S2 class) using the wasted steam energy coming from the primary turbines.
My guess is turbo compounding is the next best thing... The new Detroit Diesel DD15 I6 is getting a 50hp boost from the compounding.

MikeyB

 

Raising boost won't change the effective compression ratio even with no increase in fueling. What you are increasing is the MASS of air compressed, not the RATIO to which is is compressed. They are not the same.

For example, what happens in a gasser hot rod if I simulataneously increase compression as I go to a more aggressive cam, such that my "cranking compression" is constantly at 200psig? At engine cranking speed, the late cam timing is capturing less air, but compressing to a higher ratio, hence you see the same reading on the compression tester. This is an example of constant compression pressure at increased compression ratio.

The opposite thing is occuring when you increase boost in an idling diesel. You are increasing the effective compression pressure without increasing the ratio.

I hope I'm not being pedantic on that point, because the effect in reality is very similar to increasing the compression ratio--only you don't get the corresponding increase in expansion ratio that a real compression increase generally gives. This is typically why higher compression is more efficient. Higher compression also increases breathing efficiency as the engine will "suck" harder on the ports on the intake and "push" harder on the exhaust as the rate of change in cylinder volume is more dramatic.

As an interesting aside, the Miller Cycle engine (featured in the Mazda Millenia S a few years ago) divorced compression and expansion ratios by manipulating cam timing. It is a high compression engine that closes the intake valve late and prevents knocking, but allows larger expansion ratio-- thus giving both power and mpg.

The amount of water you can inject into the exhaust at idle and low boost conditions is limited because the cool temps and lower mass flow won't vaporize as much water. As boost comes up, you can ramp up the injection rate exactly as you would with injecting water in the intake.


What effective will water injection in the exhaust have upon the engine's ability to breathe? Remember that the expansion of the steam will increase drive pressure, and this will increase engine back pressure as well.

So the critical question becomes whether or not the higher boost levels allowed by exhaust-injected water increase boost more than they increase backpressure.

I think they odds of a system successfully injecting water into the exhaust improve with a largish turbine that can be exploit the higher drive energy.

Justin

 

Conservation of energy says that the turbo spooling up and compressing intake air comes from a temperature drop in the exhaust gas passing through the turbine. All of the energy needed to compress the intake air is a result of this temperature drop. Injecting liquid water into the exhaust manifold and converting it to steam will consume a massive amount of heat energy when the liquid changes state to a gas. I am guessing that although the turbo will see a higher exhaust gas + steam volume, its temperature will be much lower. Perhaps a way to measure how much this water injection helps is to measure the exhaust temperature after the turbo. If the temperature is higher without the water injection, then it is helping by capturing more heat energy. If the exhaust temperature is about the same with and without the water injection, it didn’t help.

It has been decades since my study of thermodynamics and steam tables, but one thing that was drilled into to me was there is no free energy. Thanks for the good discussion.

 

It is the entire engine system that must be analysed.

Concerning the aspect of capturing engine waste heat. I don't think you can look stictly at any single factor. For instance, the example of injecting water post combustion to cool the combustion chamber and to increase turbo output. This in itself does not increase engine Hp or lower BSFC as would injecting the water pre-combustion.

There already is sufficient energy (waste exhaust heat) available for the turbo to utilize. Actually, more than enough. Adding water / steam to the exhaust to me seems like it would simply allow the turbo to build more boost. But there has been no efficientcy improvement other than that of a smaller engine producing more Hp. What is not to like about that?

But the BSFC will not drop. The maximum engine Hp could increase, because the capabiliity for increasing the maximum fueling rate has increased. Again, assumimg that temperatures can still stay acceptable.

I think you could increase the efficientcy of the turbo / CAC system and achieve the same results for probably cheaper. None the less, there would still be a greater pool of energy for the turbo to tap into.

You have considered re-capturing the waste energy to improve BSFC. This is a sound principle for any heat engine the requires energy rejection to operate. The hard part is capturing a usable portion of the waste energy. As you are aware, the two places of capture are the exhaust and cooling system. The energy is there, it is just hard utilize it in a cost effective way.

There is an engine design that actually does this. Altogether not sure how it works, but call it a combustion / steam piston hybrid engine. The engine has no circulative water coolant system, but instead uses block and exhaust (normally wasted) heat to heat water. In that process it cools the block and combustion chamber components. This hot water / steam is injected directly into the combustion chamber as a seperate power stroke. In the fashion of a steam locomotive. The process cools the engine as well as provides more usable Hp.

The engine kind of does something like:
Intake
Compression
Combustion -> Power
Exhaust -> Gas
Hot Water / Steam Injection -> Power
Exhaust -> Steam

This engine has a direct BSFC improvement.

Jim

 

Yeah, the effect is the same almost. The terminal compression temperature would be lower for boosted lower CR as compared to unboosted higher CR. Mostly due to CAC I would think.

Jim

 

IMO, it's also helpful to discuss from a process control point of view - it magnifies the rather large centers of "waste" in ICEs.

 

After rereading the 3 pages of posts again, let me go out on a limb and guess how this might work. With compound turbos it may be possible to design a system to flow liquid water through a pipe thermally connected to the exhaust manifold to create steam. Water would be forced through this pipe with a pump similar to the ones used for water/meth injection. The resulting steam would then be fed into the exhaust entering the primary turbo effectively increasing the mass flow and spool. The problem I see with this is the complexity of how it is controlled. You would have to monitor the manifold temperature to assure no liquid water would enter the turbo under any condition. The added TIP of the primary turbo will increase and cause more backpressure, but that tradeoff might be OK.

Am I close or is the limb I’m standing on beginning to crack?

 

Yeah OK sorry. I need to read a bit better. For some reason I read that as post combustion vice post combustion chamber.

I was thinking that you were considering injecting water directly into the combustion chamber on the exhaust stroke. To act as a direct coolant for the combustion chamber and to increase turbo output. In effect, partioning a greater percentage of heat out the exhaust to be used the by the turbo. Vice the same amount of heat simply being dumped out by the radiator doing no work.

So you were thinking something along the lines of a marine type exhaust system. Where counter flow water cools the exhaust components via double wall exhaust piping. The hot water then ported directly into the exhaust path itself. That correct?

I am not sure, but my gut tells me that you would be better off with higher exhaust temperature / pressure vice greater mass flow at a lower temperature for the turbine efficientcy. This is mostly due to the design and rpm of the turbine wheel. Although steam power augmentation is viable and used for industrial power turbines to increase heat rate and power output capability. Basically a jet engine is used as a gas generator and the steam is admitted into the gas stream upstream of the power turbine. Those turbines run in the 3600 rpm range and do experience accellerated blade erosion as a side effect of the process. I am guessing because of the design primarily being a gas turbine vice a steam turbine, not sure though.

Probably the water spray will not decrease the exhaust pressure much, but it would surely drop the temperature. Like you said above at that end the day, you would have to ensure some degree of super heat in the steam component of the exhaust. I don't think the turbine would tolerate very much condensate / water impact. Plus the pressure pulsation of the exhaust may lend to inherent problems. But thinking forward into that idea, a lower speed turbo with a condensing type turbine might be what the doctor ordered.

All that being said, I have wondered a few times why this is not actually being done today. I mean a injector design that would actually inject fuel and water. Possibly with a few millisecond delay for the water injection to allow combustion to initialize and / or stablilize. Fuel on the inside spray cone and water on the outside spray cone. I suppose cost and complexity of the equipment needed as well as the requirement to carry onboard fuel and water vice just fuel. The water purity might also need to be very high. I would think that the NOx / combustion temperature control becomes a snap by controling the water injection rate. You might be able to ditch the urea SCR systems.

For the urea set-up, IMO Urea or Yuck are both equally bad four letter words....

Jim