All else being equal (including fueling), would there be a power or efficiency penalty paid to have a CTD make more boost than it "should".... for instance, cruising down the highway you had 15psi boost where you'd normally have 5psi boost at that load/fueling level?

 

There is a specific stoichiometry that you want to run. At this air fuel ratio, you are burning 100% of your fuel and being as efficient as you can be. Once you start introducing more air, it makes the reaction harder to drive and everything gets cooler. If you don't have enough air, all of the fuel is not burned(black smoke).

I don't know whether our trucks are running right at the most efficient point when we drive down the highway or not. I doubt that they are pushing too much air but they might not be pushing enough.

Remember that boost is not a good indication of airflow. It is a better indication of backpressure in the intake.

 

Actually, diesels perform well over a wide range of fuel/air ratios. There are distinct stochies for power, smoke (or lack thereof) and emissions. Compression-ignition engines essentially run lean most of the time, anyhow, so I'm not sure running leaner would negatively impact performance from an A/F perspective.

However, the extra work required by the engine to compress the extra air (from the extra boost - or airflow) is in question... wouldn't the extra boost help push the pistons down on the intake stroke, thus offsetting some, most or all of the extra work required on the compression stroke?

 

No, it would be neglidgable. Many would say that the piston is still drawing the air in, even under pressure, but I'm not really sure if that's how it works. You can pressurize a cylinder to an incredibly large amount of pressure and never move the piston a fraction of an inch (similar to how one often goes about changing valve springs without removing a cylinder head.

 

The air is moving into the cylinder because of a pressure differential. By increasing the pressure at the inlet, the differential is larger and more air will make it in before the intake valve closes. Since the heads on these motors are restrictive, you can never really "push" the piston down but there will be less resistance to it being pulled down by the crank if you have a higher outside pressure. However, I think that this benefit would be too small to be important. Remember that the turbo will steal even more energy from the exhaust than the amount that will be recovered in the piston going down easier if it even matters.

It is true that diesels have a wide range of air/fuel ratios where they will run well. They run lean quite efficiently but the efficiency will drop off as you try to make them more lean. Also, producing that much boost will take energy.

If you look at a P-V diagram for the diesel cycle, you will see where the large efficiency losses are. There is a huge loss because we don't completely expand the air in the cylinder before opening the exhaust valve. The turbo helps to recapture some of this unused energy. The other big efficiency lies in how all of the corners of the graph are rounded of so there is lost area. The best way to get this energy back is to have the valves open and close faster. This is the reason that vehicles are going to more valves that are smaller because they are faster. While this does not directly relate to the question, it shows how proper delivery of air is important(ie cold air with fast acting valves).

 

I consider any energy the turbo recovers from the engine's waste heat in the exhaust to be beneficial.

The additional boost would be produced by excess waste heat rejected by the engine.

To qualify, I'm asking what the relationship is between intake boost vs. required crankshaft compressive power... direct, inverse, proportional, indirect???

 

I've heard the lower the boost the better the mpg because it means the engine is not working as hard. I dont know, just what I heard.

 

When I try running my truck at or below 5 psi of boost (~4000' elevation on a flat road in 6th gear) it gets great mileage but it will not go faster than 65 MPH.

You have to create the heat in order for the turbo to use so I'm thinking your efficiency would be lost in the amount of fuel consumed.

 

Here's my amateur perspective (keep in mind my "engineer" training is a couple generic undergrad classes and that's it-- I majored in political science).

Let's begin by asking what drives the turbine? It's enthalpy-- in plain language, a combination of heat and pressure. The heat part is basically free energy, and the primary source of a turbo's efficiency. But the pressure part is NOT "free lunch"-- it's a direct source of pumping losses to the engine.

Keeping this in mind, let's look at another pivotal question: do cooler EGTs make more power? I believe the answer to this is NO, they do not. Once you have cleaned up all your smoke and completely burned every last drop of fuel, you have extracted the power it can offer. How efficiently you've done this is a function of engine timing, cam timing, etc. The amount of excess air does influence this value a little-- the more excess air you have, the less timing you need and therefore less "negative work" is done and you'll improve efficiency. But I suspect (no proof) that the modest timing retard allowed is more than offset by the huge pumping loss it would take to generate a huge surplus of air.

So this leads us to the conclusion that EGT control is not for more power, but rather to keep the engine alive. A diesel can make power with EGTs that will melt the engine. In other words, you hit the physical engine limits before you hit the point where the engine is making LESS power due to air starvation.

So a theoretically perfect turbo would not make any boost until EGTs hit 1200 or so, then it would add enough air to keep EGTs constant at a survivable level.

In other words, a "max efficiency" turbo would make just enough boost to keep EGTs from meltdown and no more.


But you'll never even come close to such a setup, nor would you even want to try. Firstly, the operating parameters of these engines vary so widely that an "optimized efficiency" setup is pretty much useless and undriveable. Where would you want this "max efficiency" to occur? How much power at this point? If I want with a stock 24V HO numbers and wanted 245hp at 2700rpm with "max efficiency", I'd only need 14.7psig of reasonably cool boost. Moreover, the "max efficiency" for this small amount of boost would be a large, loose turbine and housing that wouldn't spool before 2K rpm at all. Forget daily driving or towing.

Getting back to the original question, the relationship between crankshaft power and excess boost larglely depends on the given operating conditions relative to the operating range of the components. If you are pushing a 35/40 hybrid with the mismatched undersized turbine wheel to a boost level of 45psi or so (off the hx40 compressor map), then I'd say that extra boost is costing you HP, and the more extra boost your run, the more power you lose.

As to the characterization of the relationship between intake boost and required crankshaft power, it's sometimes proportional, sometimes directly proportional, and sometimes inversely so.

For example, the first 10hp of turbine hp delivered by the turbine comes almost for "free". The loss of engine hp due to turbine restriction is minimal. Then as boost rises because the turbine is extracting more power from the engine [not JUST from the exhaust, but also the engine in the form of increased backpressure] you will see the marginal increases in turbine power delivery get smaller and smaller and the marginal increases in engine power loss get larger and larger. In other words, as the boost comes up, it's robbing more crankshaft power, and the power robbed goes up faster than the boost increase does.

So imagine a race (here comes pride on the backstretch, heartache's rolling to the inside)--- sorry.

Anyway, imagine a race where you have boost starting out with a big lead, but engine power loss has a much faster car. As boost comes up the race is on and if boost increases far enough the "fast car passes the slow one".

So I'm of the mind of that want just enough air to keep EGTs reasonable and no more-- and as much spoolup as you can get. These are not going to be combined effectively-- a good-spooling turbo will always have too much "excess air" and be inefficient with not only the overboosting, but also with the fact that so much gas is bypassed around the wastegated turbine.

Sorry this post is so verbose and poorly written (even for me) but it's late and I'm not thinking well.

Justin

 

The "extra work" to compress more air is basically returned when the piston goes back down with correspondingly more force. It's generally a wash, albeit one that's more stressful on parts (hg) at higher levels.

Say you have 10:1 compression and start with 30psia at the bottom of the stroke. You'll have 300psia at the top, and 30psia when it goes back to the bottom. This is what I consider a "perfectly elastic" process- you get back 100% of what you put in, just like a ball the bounces up to the exact height it was dropped from.

If you want to talk cylinder pressures and ratios, you need to use EFFECTIVE compression and expansion ratios, which are determined by how late the intake valve closes and how early the exhaust valve opens. So even though an HO 24V is 17:1 static compression, effective compression might be only 14:1 or less. Expansion ratio varies similarly. Remember that camshafts don't have to be ground symmetrically, so there's a good chance that the valve events are different on the way up and on the way down.

For example, just because the intake might close 75º Before TDC, doesn't mean that the exhaust will open 75º After TDC. Intake valve closure is timed to tune a specific RPM of peak tq. The later it closes, the higher the peak tq rpm will be (ram effect of incoming air). Exhaust valve opening is a balance between 1) extracting as much useable heat and pressure from the combustion event as possible, and 2) allowing enough time to "blow down" the cylinder.

I'd personally open the exhaust valve on a turbodiesel pretty early and sacrifice some of the energy extracted from combustion. This, because a high compression engine doesn't need very long to "blow down" (most of the pressure expansion happens earlier in the downward travel), and we have a turbocharger that will recover a portion of the wasted charge energy.

Again, crappy post but it's pushing midnight and I am spent. Long day at Waikiki, y'know

 

I think you're thinking more about gas engine technology than diesel. Since the fuel is not mixed to a certain ratio in diesels the idea of rich and lean means little. It only really matters when you can't get enough air in to burn the fuel needed to make the power you're after. A certain minimum amount of air is required on each compression stroke to raise the temp enough to cause combustion. This amount is generally way more than needed to burn the fuel such as when idling or at light throttle. In theory, all the fuel gets burned and the fire goes out with oxygen left over. At full throttle or full HP there may be excess fuel and black smoke. There is excess fuel but the power is high. These two conditions could be described as rich and lean but the actual burn is still going on at approximately the right ratio. It's more useful to see it as enough air or not for the power you want.

It seems like the best efficiency would be the lowest boost that would still produce the required power with little smoke. Some smoke will always be present because of imperfect atomization and cold surfaces.

I think my '93 was more efficient than my '04 at a highway speed of about 60 MPH because it seemed like it did not need boost to go that fast so it was breathing easier. But the '04 has boost all the time and a very restrictive turbo. The '04 probably runs cleaner at that speed because it has a larger excess of air on each power stroke. My TDI engine might be more efficient than either one if it was pushing the Dodge down the road at 60 because it would have a lot less mechanical friction, but on the other hand, it would have very high boost at that HP level.

Naturally aspirated engines are all boosted to 14.7 PSI atmospheric pressure. Boost from the turbo just adds to that natural boost to make a small engine act like a big engine or a poor breathing engine breath more and therefore be able to burn more fuel and produce more power. I'm really glad we don't have to carry around 1300 cubic inch diesel engines in our trucks just to get a clean 400 HP.

It doesn't matter whether you think of boost as air flow or restriction in the intake. Either way it's the pressure that causes the air to flow through the engine and overcome the restrictions in the system. Flow gives the ability to make more power. Boost causes flow and is easily read on a gauge. When you have more air in the cylinder of a diesel you have higher temps from compression so it doesn't make the reaction harder to drive. If anything it would make it more violent or the ignition more immediate.

 

Justin, I concur with your balance-of-power analysis of the required mechanical compression power vs. charge-air pressure, with the caveat of including frictional losses.

The engine wouldn't be "overboosted" in an absolute sense (45psi ), just "overboosted" in a relative sense (15psi instead of 5psi at cruise).

Again, the extra boost would NOT come from fueling, but rather the engine's otherwise lost waste heat...

While the relative rise (and eventual convergence) of the boost-raised power production and boost-raised power consumption curves can be assumed, how much relevance would they have to off-idle, part-throttle engine operation?

The reduction of boost threshold and the increased rate of boost rise are no-brainers; however, I'm trying to get a handle on whether or not full-time operation is worthwhile (CCC issues). For every knowledgable diesel head that takes one side, there's an automotive engineer with more degrees than a NY law firm who believes the opposite!

 

So XLR8-- you're saying for example what would be the difference in BSFC between a regular stock charger pushing 7 psi on the hwy, and a might tighter housing charger that would give you 15psi under the exact same conditions?

By "overboosted", I understand you to mean that we are running "excessively lean". (I almost typed excess excess air, but that's pretty dumb of me).

If I understand your question correctly (not likely), you asking if having lots of surplus air under light load conditions will negatively affect engine efficiency.

I simply don't know. I suspect that it will. One analytical technique I use besides "exploring the opposite" is "taking the principle to the extreme". What if I wanted 300º EGTs cruising empty on the interstate at 65mph? That's a lot of excess air, no? You might have 20psi at 65mph!

I suspect this would have a huge pumping loss, and therefore a big negative on BSFC.

Anecdotally, this would seem to be true. Emissions considerations are having diesels run leaner and leaner to reduce peak combustion temps and thus NOx. We know that smogged-out diesels do not deliver the mpg of their less sophisticated predecessors, given the same operating conditions. EGR is used for the same reason: the introduction of inert exhaust gasses slows the rate of combustion by making oxygen less dense and causing a slower flame propagation. The slower flame propagation reduces peak temps and pressures, and hence NOx.

Ask yourself this: what if I connected the crankshaft of my engine to a supercharger that was massively overdriven to generate a lot of excess air? Efficient? Hardly. (even you had a centrifugal compressor in place of the less efficient roots style). The Top Fuel guys consume more than 500hp just turning the supercharger! (I wanna say it's actually a lot more than that, but I don't remember OTTOMH).

Going back to the enhalpy thing of temp and pressure, I'd say you want to drive your turbine with heat and not so much pressure, because while the heat is waste heat, the pressure is NOT- because of the fact that the exhaust restriction increases pumping loss.

What is CCC? (I always think of that as Campus Crusade for Christ, but that doesn't fit your context)

 

It seems to me that having higher boost than needed for a given HP level would lower efficiency by not only having higher pumping losses but also by the increased mass that must be heated with the available fuel on each power stroke. Heat is what increases power so If there is more air to heat with a given amount of energy the temp rise will be less. The exhaust would look cooler but the actual amount of energy coming out the exhaust might be higher than if there was less air in the cylinder on each firing. It seems that it would have the same net affect as EGR, a cooler combustion temp. In this case caused by a larger mass to heat rather than by slower flame propagation. Am I way off track here?

As I mentioned earlier, my '04 seems to always have boost and I can't see the benefit at light loads, except maybe emmisions and/or throttle response.

John

 

I mean everything else being equal - right down to the stock HE351 charger... the only difference is the extra drive pressure resulting in artificially high compressor boost.

By "overboosted" I mean boost in excess of what normally would be available in a particular engine load scenario, or - even more excessively lean than usual.

We're not after cooler EGTs, though they can be considered a fringe benefit by some (not something I subscribe to) - our focus is enhanced turbine inlet pressure, which generally is accepted to be thermal energy-driven expansion, but can also be produced by other means.

The emissions aspect is another thread entirely; most people don't understand the actual combustion process well enough from the chemical reaction point of view to realize how it is affected by EGR, forced-induction or water injection, to name a few. I don't see much of a case for EGR's contribution to significant pumping losses.

Returning to enthalpy, thermal energy alone won't do anything to a turbo except change it's color... it's the expansion of the fluid medium (in this discussion exhaust gas) caused by the thermal energy. Only pressure will turn the turbine, and while it's certainly true that using a portion of the engine's waste heat in the exhaust tract is an efficient way to create that drive pressure, a large part of TIP is a direct result of cylinder blow-down.
After all, exhaust gas only has ~ .25BTU/lb/*F of thermal energy.

p.s. CCC usually means "Certifiable Cummins Christian" but in this case stood for Command, Control & Communication - there's differing levels of complexity required if the system is operated full time in passive mode or on-demand only in active mode.