N/A vs Boosted Power? 3

[PoorManagement]

I'm having trouble understanding how an engine can produce more power boosted vs naturally aspirated.

I'm assuming that an ideal NA motor - let's say a 2.0L making 220hp is doing so at the limits of the structure of the engine - for arguments sake, with cylinder pressures of 120 bar.

Now you add boost, and the engine can make 400hp.

But the structural limit of the engine is still 120 bar before parts fail.

Is it possible that peak cylinder pressures are roughly the same? (I'm missing a relationship between peak cylinder pressure and mean effective pressure - work done as the volume expands)

How is the additional power achieved while keeping peak cylinder pressures at a level that does not hurt the engine? Spark retard reduces the absolute peak pressure, but the pressure for the entire combustion cycle is higher overall? (Higher MEP?) Is this my answer? I think it is...

I'm assuming that no other physical changes are made - same pistons, rods (so same compression ratio), block, heads, etc.

Assuming the boost device provides additional air at a reasonable temp (intercooled), and the fuel system delivers the appropriate amount of fuel for the available air...


Thanks!

 

[Lou Scannon]

If the structural limit is indeed 120 bar, it is still possible to raise the power density within this limit, simply through delayed combustion phasing; albeit with a penalty in BSFC.
There are also thermal loading limits to consider, which may get in the way of the above strategy.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[BigClive]

Any power over 220hp progressively hurts the engine.

 

[GregLocock]

That's only true IF the peak cylinder pressure was a design limitation. I'd agree that generally that may be the case, but experience suggests that substantial increases in torque can be achieved without significantly affecting durability, or if you prefer, original engine designs have a fair bit of structural overdesign in them with respect to bmep. If bmep were truly a limiting factor then increasing the torque of the NA engine by 50% would reduce the engine life by a factor of 3 (rule of thumb for durability).

Part of the reason that turbocharging in particular doesn't wreck engines is that the pressure rise rate tends to increase more slowly than bmep, for reasons I have long forgotten, so the 'impact' type excitation doesn't increase as much as you'd expect.

Cheers

Greg Locock


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

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

 

[MatthewDB]

If you boost the heck out of the engine at lower RPM where the volumetric efficiency is low, it will indeed break the engine. As the speed increases, the volumetric efficiency drops. The boost compensates that volumetric efficiency and keeps the torque up. A turbo properly matched to the engine does this automatically because the turbine won't produce a lot of power for the compressor at low speed. For a supercharged engine, something has to regulate the boost or reliability will be affected.

The big gain from boosting is the torque increase at higher RPM; it shifts the peak horsepower point up. Remember power is torque times rotational velocity. Having the same torque at higher RPM is more power.

 

[Tmoose]

Hi Poormanagement,

earlier you said "I'm assuming that an ideal NA motor - let's say a 2.0L making 220hp is doing so at the limits of the structure of the engine - for arguments sake, with cylinder pressures of 120 bar"

====================

Why would you assume that?

Although "making it through the warrantee" is an important requirement, manufacturers of successful products had better do a bit better than that.
In 2006 Toyota's basic Warrantee was 3 years 36,00o miles, and the powertain warrantee was 5 years 60,000 miles. If Toyotae had the rep for engine trouble at 65,000 miles I doubt they would have outsold Ford in the US that year.

http://usatoday30.usatoday.com/money/autos/2006-08-01-sales_x.htm

I question if such pin-point engineering calculations even exists. If they did, it seems like it would have gone a long way to creating a world where there would be NO warrantee failures like the famous Ford Taurus automatic AX4S transmission pre 1995 aluminum clutch piston failure. (Most of those transmissions did survive beyond 60,000 miles). I find it interesting that most articles blame the failures on the aluminum piston's "cheapness", without any mention of how the replacement steel piston design or material properties differ.



There are tales that In the early days of drag racing hot rodders in emergencies would replace blown up engines with junkyard engine short blocks and compete successfully, for a few runs anyway. Some of those engines designed and developed without benefit of computer simulation were pretty stout.

Similarly, Every few months of 2014 it seemed there was an article in Hot Rod magazine, etc about taking a used V8 engine, sometimes from a junkyard automotive recycler, sometimes without even disassembling and freshening the lower end, installing a supercharger on it, and squeezing 800 plus HP out of the poor thing on some dyno somewhere, at least long enough for a dyno sweep.

I'm thinking that original designs these days initially focus on things like resisting deflection under load and resistance to thermal cycling, and packaging concerns, so that straight "strength" may even be carried along nearly for free with analysis for resistance to fatigue or something similar.

I especially enjoy this comment by Unbrako on page 64 of their Engineering guide for the simple sounding calculation of thread engagement to resist tapped hole thread "stripping.".

http://www.unbrako.com/docs/engguide.pdf

"Attempts to compute lengths of engagement and related factors by formula have not been entirely satisfactory-mainly
because of subtle differences between various materials. Therefore, strength data has been empirically developed from a series of tensile tests of
tapped specimens for seven commonly used metals including steel, aluminum, brass and cast iron."

 

[BrianPetersen]

The error in the original post is the assumption that the normally-aspirated condition will be "at the limits of the structure", because assuredly that is not the case. Most real world production engines are overbuilt - the only question is "by how much".

But even without that ... It will generally be found that where there is a naturally-aspirated and a forced-induction version of an otherwise-similar engine, the forced-induction version has a lower compression ratio, and this offsets the increase in peak cylinder pressure that would otherwise occur.

The forced-induction engine will make more power (despite the loss in compression ratio) because it will have higher pressure throughout the complete operating cycle, not just at the peak.

 

[Lou Scannon]

Part of the reason that turbocharging in particular doesn't wreck engines is that the pressure rise rate tends to increase more slowly than bmep, for reasons I have long forgotten

Probably because as [at the design stage] boost is increased, compression ratio is decreased, along with spark advance, typically; precisely to avoid problems with combustion knock and possible excessive peak cylinder pressure.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[Lou Scannon]

...and if you mean turbocharging is less severe in this regard than crank-powered supercharging, you are right, because for a given net power increase, mechanical supercharging requires more gross (think IMEP) power increase, for reasons which should be obvious.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[tbuelna]

In theory you can increase power without increasing peak combustion pressure at a given set of operating conditions by increasing the brake mean effective pressure. Just expand the area under the curve.

 

[BrianPetersen]

^ Just bear in mind that doing so is not without side effects. Delay combustion until after the peak compression, means that more of it effectively happens at a lower compression ratio. Thermal efficiency goes down, exhaust temperature goes up, exhaust valve and piston heat loading goes up, cooling system heat loading goes up, etc. No free lunch.

Ask Ford Ecoboost truck owners how their fuel consumption is with a trailer in tow.

 

[PoorManagement]

Thanks all for the replies.

What I was trying to get at was, making the assumption that an engine was at its structural limits, was it possible to make more power with the same (unchanged physically - ie same compression ratio, same strength rods and pistons, etc) engine boosted vs NA.

I'm assuming that the engine is at its design limits (1.0 safety factor).

I get that my assumptions aren't practical - it's a theoretical question.

And I think the answer that best explains, for me, the physical situation, is that engine power is a function of area under the curve of the PV loop, more than just peak cylinder pressure.

That is to say that higher total BMEP can be achieved without raising the damaging (per my initial assumption) total peak cylinder pressure which results in higher engine output via delayed ignition timing, which, as was stated above, really reduces the effective compression ratio.

Anyway - I think my curiosity has been sufficiently satisfied!

Thanks all!

 

[tbuelna]

Just to be clear, what I proposed is theoretically possible. But in practice it is likely not possible to produce anywhere close to the level of power increase (ie. 400 vs 220) you describe in your OP. The peak power gains from optimizing combustion and doing more of the compression/expansion work using turbo equipment, without increasing the structural loading on the engine components, would be marginal.

 

[BigClive]

I would thought that the main problem in going from 220hp NA to 400hp turbo would be from the greater amount of heat released rather than problems from the greater cylinder pressures etc.

 

[gruntguru]

The first problem for a 100% power increase is detonation and the usual remedy is reduced CR which also reduces peak cylinder pressure and mechanical loads. Fortunately most cars with 100% power increase can only be driven at full power for very short bursts so the cooling system catches up with the extra heat stored in the metal and coolant between bursts. In addition, cooling systems are usually designed with a healthy margin.

If continuous duty at the new power level is required (e.g. towing) the cooling system would obviously need to be upgraded.

100% power increase needs 0.8 to 1.0 bar boost or 0.6 - 0.8 bar if inter cooled. The lower boost and the intercooler itself also reduces the heat load on the cooling system.

Engineering is the art of creating things you need, from things you can get.

 

[ivymike]

What I was trying to get at was, making the assumption that an engine was at its structural limits, was it possible to make more power with the same (unchanged physically - ie same compression ratio, same strength rods and pistons, etc) engine boosted vs NA.
Yes, it is possible, but you'd be going about it wrong. See Gruntguru's post.

And I think the answer that best explains, for me, the physical situation, is that engine power is a function of area under the curve of the PV loop, more than just peak cylinder pressure.
It's not true to say that PCP has no relationship to peak power, since there are limits to how quickly/slowly combustion can happen and how early/late you can start it ... but if you could shape the cylinder pressure curve however you liked, then PCP would have no relationship to peak power [except in the bizarrely contrived instance where PCP was applied throughout the entire expansion stroke and PCP was more-or-less equal to BMEP]

 

[PoorManagement]

Thanks all for the input.

I agree that cylinder pressure isn't the only concern if I were actually trying to accomplish the posed question.

But I'm not really trying to do this. So cooling, intercooling, other structural considerations weren't really the question I was trying to answer.

I agree, though, that many things would require consideration to accomplish my theoretical question.

The numbers stated were just to help picture the question better. Hopefully it didn't bog down the thought process too much. For me, it helps to have some 'for instance' type numbers to think thru the situation. My close to 100% power increase may have sent you guys thinking about all the other trouble one might get in to when attempting such a task. That wasnt my intent.

Apologies if I wasn't clear - and thanks again for the discussion.

 

[gruntguru]

If peak cylinder pressure must not increase and CR must stay the same, you will be able to increase the boost while retarding the ignition timing to limit peak cyl pressure. This will produce the "wider" PV diagram referred to. Unfortunately efficiency will suffer (late combustion) and you will certainly not get 100% power increase. Intercooling will help by increasing the charge mass for a given compression pressure.

Engineering is the art of creating things you need, from things you can get.

 

[pwildfire]

I think the answer to your question is this:

For a given engine which operates naturally aspirated at a given peak CP, and with no additional limitations:
If we add a turbocharger which is at peak efficiency at this mass flowrate, and reduce compression ratio such that peak CP is the same, we will find that pumping losses will be reduced, and therefore slightly higher system efficiency will be attained, with slightly higher power output.

The turbocharger is a net gain because much of the energy driving it comes from heat and impulse energy in the exhaust. In other words, the backpressure added is not proportional to the intake pressure generated, therefore PMEP is lower. IMEP can also be increased a bit because since the engine is operating at a lower pressure ratio the valve timing can be opened up a little bit. As such, BMEP goes up on both fronts, and we generate a little bit of additional tire smoke!

For the case of a mechanical supercharger, the IMEP case is still true, but the power necessary to drive the compressor is lost (basically as FMEP), and is roughly proportional to the PMEP improvement divided by supercharger efficiency. In this case increased IMEP and decreased PMEP is mostly cancelled out by increased FMEP, and we end up with a small net gain, if any.

In addition, if we have the ability to adjust boost pressure (as well as ignition and fuel) based on operating condition, we can limit peak pressure more easily than if we have a high compression naturally aspirated engine. On a side note, we also widen the torque curve so that lower peak power is necessary for the user to feel the same acceleration.

 

[Lou Scannon]

For a given engine which operates naturally aspirated at a given peak CP, and with no additional limitations:
If we add a turbocharger which is at peak efficiency at this mass flowrate, and reduce compression ratio such that peak CP is the same, we will find that pumping losses will be reduced, and therefore slightly higher system efficiency will be attained, with slightly higher power output.

Your hypothesis seems to be based on external volumetric efficiency, and is probably correct as far as that goes. And, in principle, the resulting greater mass air flow will yield greater brake power. But, in the given circumstances, the combustion efficiency is bound to fall drastically as expansion is diverted from the power cylinder to the turbine, and completely negate the gain in external VE. I would appreciate seeing a citation of a real world example or hypothetical case with quantitative analysis that contradicts my contention.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz