Talking to a turbo expert

[WhiteBlue]

I prefer to look at a ball park figure of 10% more shaft power by turbo compounding over the turbo charged engine, irrespectively wheather you use an electric or a CVT system. This is what it is going to do to our thermal efficiencies:

conventional V8 engine 29%
direct injected turbo engine 33%
turbo charged and turbo compounded 36.3%

Ringo's figures suggest that would be realistic.

[WhiteBlue]

I'm bringing this up for a reminder. Please remember that we did not know the precise rules that have been published since on the FiA web site. The rules change some of the estimates. Particularly the way the MGU-H can feed electric power directly to the MGU-K without losses occurring for dual conversation in and out of a battery. I think the general calculations of the thermodynamic aspects are valuable. Perhaps we should revisit all the numbers and update this thread?

[olefud]

I prefer to look at a ball park figure of 10% more shaft power by turbo compounding over the turbo charged engine, irrespectively wheather you use an electric or a CVT system. This is what it is going to do to our thermal efficiencies:

conventional V8 engine 29%
direct injected turbo engine 33%
turbo charged and turbo compounded 36.3%

Ringo's figures suggest that would be realistic.

Why is the turbo engine more efficient than a comparable power/featured conventional engine? It would seem that the conventional engine would have less waste exhaust heat even after the turbine in the turbo engine.

I’m not questioning the figures; just wondering why it’s so.

[WhiteBlue]

Why is the turbo engine more efficient than a comparable power/featured conventional engine? It would seem that the conventional engine would have less waste exhaust heat even after the turbine in the turbo engine.

That is not true. The exhaust gas stream has the most energy in the naturally aspired engine. Next is the simple turbo engine that takes out a part of the energy to compress the charge air. Finally the turbo compounded F1 engine takes it one step further by fitting an over sized turbine compared to the compressor power demand. That takes even more energy out of the exhaust gas than the other two types of engines. The additional power is fed to the electric motor (formerly KERS). Because you have less waste heat and kinetic energy in the exhaust gas you make more power from every drop of fuel. And that is what engineers call higher thermal efficiency.

[olefud]

Why is the turbo engine more efficient than a comparable power/featured conventional engine? It would seem that the conventional engine would have less waste exhaust heat even after the turbine in the turbo engine.

That is not true. The exhaust gas stream has the most energy in the naturally aspired engine. Next is the simple turbo engine that takes out a part of the energy to compress the charge air. Finally the turbo compounded F1 engine takes it one step further by fitting an over sized turbine compared to the compressor power demand. That takes even more energy out of the exhaust gas than the other two types of engines. The additional power is fed to the electric motor (formerly KERS). Because you have less waste heat and kinetic energy in the exhaust gas you make more power from every drop of fuel. And that is what engineers call higher thermal efficiency.

Maybe under some unstated parameters. But what about the example of the two engines sized for the same power rating, one power cycle with the conventional engine at 100% volumetric efficiency and the turbo at 150% VE? The larger displacement conventional engine will have greater volume for power stroke expansion thus utilizing more of the heat energy. The turbo will finish work expansion with higher residual cylinder pressure and waste a bit more with an earlier exhaust valve opening for blowdown to relieve the higher pressure prior to the exhaust stroke.

The energy reclaimed by the turbo is not useful energy. Turbocharging has advantages but thermo efficiency wouldn’t appear to be one of them. The whole purpose of compounding is to reclaim this wasted heat energy.

[Tommy Cookers]

Because you have less waste heat and kinetic energy in the exhaust gas you make more power from every drop of fuel. And that is what engineers call higher thermal efficiency.

it's what engineers call higher brake thermal efficiency, which can also be called higher overall efficiency
(not what engineers call thermal efficiency (aka indicated thermal efficiency), which has a different meaning)

this is important regarding the working and value of downsizing
reducing mechanical etc losses relative to output will improve brake TE even without needing a change in indicated TE
isn't this at the heart of downsizing ?
raising the power by raising the massflow without (significantly) raising the mechanical losses will tend to improve efficiency
as will reducing the mechanical losses etc by reducing the displacement (and/or rpm) and maintaining the massflow

[autogyro]

Ferrari dont seem to think so, at least for road cars.

[Tommy Cookers]

It would seem that the conventional engine would have less waste exhaust heat even after the turbine in the turbo engine.

That is not true. The exhaust gas stream has the most energy in the naturally aspired engine. Next is the simple turbo engine that takes out a part of the energy to compress the charge air. Finally the turbo compounded F1 engine takes it one step further by fitting an over sized turbine compared to the compressor power demand. That takes even more energy out of the exhaust gas than the other two types of engines.
you have less waste heat and kinetic energy in the exhaust gas

the Wright Turbo-Compound (that recovers a continuously-measured 18% of max power from its exhaust turbines) has a conspicuously and infamously nearly white-hot exhaust from the tubines (as in the EAA 'footage' of a recent DC-7C flight)
apparently hotter, not cooler than otherwise identical Wright engines without the exhaust turbines

[olefud]

Because you have less waste heat and kinetic energy in the exhaust gas you make more power from every drop of fuel. And that is what engineers call higher thermal efficiency.

it's what engineers call higher brake thermal efficiency, which can also be called higher overall efficiency
(not what engineers call thermal efficiency (aka indicated thermal efficiency), which has a narrower meaning)

this is important regarding the working and value of downsizing
reducing the proportion of frictional losses will improve brake TE even without needing a change in indicated TE
isn't this at the heart of downsizing ?

Generically, as opposed to F-1, IMO the most significant contribution of turbocharging is the decrease in engine mass. Friction is a bit more problematic. Friction is largely a function of area and pressure/velocity. Higher turbo combustion pressure increases ring pressure though there is less ring friction area. Turbo engines also tend to rev higher though there now appears to be a movement towards higher torque, lower revs. Lots of choices and tradeoffs.

Efficiency is of interest to me in that I’ve been doing a bit of development work to avoid the inefficiencies of the NA engine. I have had some promising qualitative results, but the comparison between turbo and NA is difficult. What’s the tradeoff between NA simplicity and less turbo mass?

[olefud]

It would seem that the conventional engine would have less waste exhaust heat even after the turbine in the turbo engine.

That is not true. The exhaust gas stream has the most energy in the naturally aspired engine. Next is the simple turbo engine that takes out a part of the energy to compress the charge air. Finally the turbo compounded F1 engine takes it one step further by fitting an over sized turbine compared to the compressor power demand. That takes even more energy out of the exhaust gas than the other two types of engines.
you have less waste heat and kinetic energy in the exhaust gas

the Wright Turbo-Compound (that recovers a continuously-measured 18% of max power from its exhaust turbines) has a conspicuously and infamously nearly white-hot exhaust from the tubines (as in the EAA 'footage' of a recent DC-7C flight)
apparently hotter, not cooler than otherwise identical Wright engines without the exhaust turbines

We may have some apples and oranges here. What were the operating power and fuel burn rates of the various iterations?
Another thought, would you compound a NA engine?

[Tommy Cookers]

the 'downsizing' argument was for decades before the car-practical turbo used to justify mechanically driven supercharging
(the best sfc for spark-ignition seem to be 0.32 lb/hp-hr and 0.34 given by Sam Heron for the race version of the Napier Lion and a 1935? P&W Wasp demonstrating high (then) octane fuel (both mechanically supercharged of course), the fuel constituents presumably a bit denser than the usual fuel)

logically IMO an NA engine is in principle compoundable
all (spark-ignition anyway) road car turbos attempt to access exhaust pulses with little raising of the mean exhaust pressure
(this is what the Wright TC does, 'free power' they showed with some plausibility)
(these pulses are used in NA performance car engines for power and increase efficiency at high powers of course)

any boosted engine (in a fair comparison) must have a lower CR/ER, so a more energetic exhaust (less energy to the crankshaft) and stronger (pressure/velocity) pulses
so the % recoverable is more (but it needs to be)
the boost maintains massflow well against the (slight?) raising of exhaust pressure

so the NA has less recovery available, and loses massflow more with raising of exhaust pressure (hence can lose efficiency)
(raised exhaust pressure helpfully reduces the large losses in blowdown pre-turbine, though (exhaust pressure being larger) this helps the boosted engine more)

the NA would best be given a recovery turbine sufficient to power an electrically-driven low pressure (centrifugal) supercharger
EDIT and/or use variation of recovery load to control massflow without throttling

[WhiteBlue]

Maybe under some unstated parameters. But what about the example of the two engines sized for the same power rating, one power cycle with the conventional engine at 100% volumetric efficiency and the turbo at 150% VE? The larger displacement conventional engine will have greater volume for power stroke expansion thus utilizing more of the heat energy. The turbo will finish work expansion with higher residual cylinder pressure and waste a bit more with an earlier exhaust valve opening for blowdown to relieve the higher pressure prior to the exhaust stroke.
The energy reclaimed by the turbo is not useful energy. Turbocharging has advantages but thermo efficiency wouldn’t appear to be one of them. The whole purpose of compounding is to reclaim this wasted heat energy.

You should start thinking in energy balance to understand the advantages of specific configurations. The higher the thermal efficiency of an engine the more power it makes from the fuel. In that regard it primarily matters how much thermal and kinetic energy is dumped with the via the exhaust gas and the radiators. One aspect already covered by TC is the frictional aspect. Turbocharged engines run on lower revs and waste less power in friction than higher revving naturally aspired engines. You need to look at the basic physical aspects to determine where your efficiencies are generated. That means comparing the main loss drivers. One other aspect of turbo charged engines is the better suitability to direct injection spray guided combustion. This type of combustion delivers the best fuel/air ratios and combustion efficiencies. But it works best at lower revs because the known injection systems are too slow for rpms beyond 9,000 rpm. There is work under way to improve the speed of injection but it is a long way to reach the 15,000 rpm the regulations allow for the new engines. So there are several good reasons to focus on the lower hanging fruits of charged engines.

[WhiteBlue]

the Wright Turbo-Compound (that recovers a continuously-measured 18% of max power from its exhaust turbines) has a conspicuously and infamously nearly white-hot exhaust from the tubines (as in the EAA 'footage' of a recent DC-7C flight)
apparently hotter, not cooler than otherwise identical Wright engines without the exhaust turbines

Naturally a mechanical device that converts energy will appear hotter than a pipe that simply dumps high energy exhaust gas. I would challenge the impression that the TC Wright engine has a higher energy state of the exhaust gas. Even if you are right with the temperatures - which I seriously doubt - you seem to neglect kinetic energy all together. Where do you think the added power is coming from if not from reducing the exhaust enthalpy? The very basic design computations of a turbine that generates power shows you that you take the energy out of the mass flow that passes the turbine. Hence the addition of the three exhaust turbines to the design cannot increase the energy state of the exhaust gas but must reduce it.

[Tommy Cookers]

Because you have less waste heat and kinetic energy in the exhaust gas you make more power from every drop of fuel. And that is what engineers call higher thermal efficiency.

it's what engineers call higher brake thermal efficiency, which can also be called higher overall efficiency
(not what engineers call thermal efficiency (aka indicated thermal efficiency), which has a different meaning)

downsizing doesn't need higher TE to work
so sucessful (ie higher overall efficiency) downsizing doesn't prove that higher TE has been reached

boosting demands lower CR and gives lower expansion in cylinder, so puts more 'waste energy' into the exhaust anyway
so it's not unreasonable to gain something from having a turbine there
charge cooling and direct injection also allow increased efficiency from the NA engine, they don't belong to the turbo

does your turbo expert opine regarding eg whether boost should be eg 0.1 bar or 1 bar or 10 bar ?
the scope in nesting a piston engine inside a turbine engine has no clear end point
(and no clear start point ? when NA (spark ign) engines have so reduced their throttling etc losses eg Fiat)
(controlled PRT load in NA engines is alternative route to this improvement)

[WhiteBlue]

downsizing doesn't need higher TE to work
so sucessful (ie higher overall efficiency) downsizing doesn't prove that higher TE has been reached

I'm not sure that we are talking about the same thing here. Downsizing according to my understanding typically involves turbo charging and a reduction of the cylinder number and displacement volume. Lets assume for the sake of simplicity that the power output, the mass flow and the radiated heat would be the same for both engines. In reality the mass flow of the turbo engine is slightly smaller due to a lower fuel consumption. If we think along that line a number of efficiency mechanisms can be identified. The cylinder reduction will give you higher mechanical efficiency by reducing friction. Let us assume again for simplicity that the engine will run on the same rpm level although it typically does not. Then we can exclude further friction efficiency gains from that. How does adding an exhaust turbine and a turbo compressor affect various energy balances we can do? That is the best way to investigate the efficiency gains the downsized engine offers.

Lets look at the balance of the turbo charger first. The compressor needs energy to compress the charge air. It gets the energy from the exhaust turbine. Actually the turbine takes out more energy from the exhaust than the compressor adds to the intake air because you have losses at the waste gate and for both turbo machines. The power for running the complete turbocharger has to come from somewhere. It comes out of the energy differential (enthalpy) in the mass flow. The mass flow of the turbocharged engine has to leave the engine at a much lower energy level than it does in the comparable NA engine that hasn't the turbine extraction or the whole contraption would not work.

At this point we can go back to the total energy balance of the engine. The energy intake with the mass flow and the radiated heat is roughly the same. So we can be certain that the turbo engine must have a higher thermal efficiency because it releases substantially less waste in form of kinetic and thermal energy at the tail pipe.

The example of the simplified downsized engine shows how the fuel saving is mainly driven by increased thermal and mechanical efficiency. It would be much clearer if we were looking at the actual figures. I would suggest to look at Ringo's thermodynamic computations of the 2014 F1 compressor and the turbine to get a feel for this.

boosting demands lower CR and gives lower expansion in cylinder, so puts more 'waste energy' into the exhaust anyway

If you look at the true exhaust at the tail pipe it doesn't. I have covered the issue above.

so it's not unreasonable to have a turbine there
there's a lot of tradeoffs

I also think it is an advantage to use turbochargers. The trade off I see mainly in complexity issues. But in terms of efficiency a turbo engine beats the NA engine.

does your turbo expert opine regarding eg whether boost should be eg 0.1 bar or 4 bar ?
nesting a piston engine inside a turbine has no clear end point (and no clear start point ?)
(when NA engines have so reduced their throttling etc losses eg Fiat)
(controlled PRT action in NA engines is one route to this)

I must admit that I do not understand your train of thought here. I opened the thread to introduce the marketing thoughts of the Garrett expert into the discussion of the turbo engines two years ago. At that time many people had difficulties with the idea of using turbo charging, particularly US people. There is nothing in the original document to answer your question, but if you put it in a different way perhaps I may be able to understand what you are driving at and continue to discuss it.