Talking to a turbo expert

[WhiteBlue]

...the BTE of an SI engine can be either improved or reduced by turbocharging.

I would think that in the majority of new design cases in the auto industry BTE will be improved in turbo projects compared to the NA alternative. I would be interested to see figures of a case where BTE was reduced by fitting a turbo. It sounds like a rather obscure case. What would be the motivation? Just brute power maximization?

[olefud]

...the BTE of an SI engine can be either improved or reduced by turbocharging.

I would think that in the majority of new design cases in the auto industry BTE will be improved in turbo projects compared to the NA alternative. I would be interested to see figures of a case where BTE was reduced by fitting a turbo. It sounds like a rather obscure case. What would be the motivation? Just brute power maximization?

Isn’t the purpose of boosting always to increase max power over an otherwise identical NA engine? An NA engine could feature identical combustion pressure. What, other than power, would turbocharging add?

[WhiteBlue]

Isn’t the purpose of boosting always to increase max power over an otherwise identical NA engine? An NA engine could feature identical combustion pressure. What, other than power, would turbocharging add?

The wide spread application of turbo charging is done in order to downsize NA engines and improve fuel economy IMO. The trend got started with diesels where turbos are practically a standard in modern design. This trend is now being followed by spark ignited petrol engines although some mainly US based users seem to deny this trend. At least the European auto industry seems to be on a crusade to downsize and turbocharge practically every new engine they introduce. This may exclude some exotic super cars and very low price cars but the centre piece of the European road car market goes for downsized designs. This leads me to the conclusion that the motive for turbo charging is more based on fuel economy than on power maximization.

[olefud]

Isn’t the purpose of boosting always to increase max power over an otherwise identical NA engine? An NA engine could feature identical combustion pressure. What, other than power, would turbocharging add?

The wide spread application of turbo charging is done in order to downsize NA engines and improve fuel economy IMO. The trend got started with diesels where turbos are practically a standard in modern design. This trend is now being followed by spark ignited petrol engines although some mainly US based users seem to deny this trend. At least the European auto industry seems to be on a crusade to downsize and turbocharge practically every new engine they introduce. This may exclude some exotic super cars and very low price cars but the centre piece of the European road car market goes for downsized designs. This leads me to the conclusion that the motive for turbo charging is more based on fuel economy than on power maximization.

I think you’re making my point. A similarly downsized NA engine is a bit on the wimpy side. In order to gain back enough power to be socially acceptable, the engine is subjected to turbocharging. But the NA engine would enjoy a cooler mixture, higher compression ratio (volumetric) and be even less weighty than the turbo.

It’s all about power.

[Tommy Cookers]

The wide spread application of turbo charging is done in order to downsize NA engines and improve fuel economy IMO. The trend got started with diesels where turbos are practically a standard in modern design. This trend is now being followed by spark ignited petrol engines although some mainly US based users seem to deny this trend.

do you understand that diesel engines have a huge surplus of air especially at low speeds, necessary to get combustion of the fuel ?
(so increased massflow/power by turbocharging (relatively) reduces the disproportionate mechanical losses of the NA diesel)
do you understand that turbocharging a diesel does not require a reduction in compression ratio ?(detonation is needed)
unlike turbocharging a SI engine, where lowering the CR is required to avoid detonation (in a fair comparison)

these are reasons why turbocharging a diesel is always advantageous
they do not apply to the SI engine

[WhiteBlue]

A similarly downsized NA engine is a bit on the wimpy side. In order to gain back enough power to be socially acceptable, the engine is subjected to turbocharging. But the NA engine would enjoy a cooler mixture, higher compression ratio (volumetric) and be even less weighty than the turbo. It’s all about power.

That's a funny way to see things. Needless to say that I don't agree. With the rest of the world I prefer more fuel efficient engines. But I see that isn't socially acceptable to you. I respect such personal choices.

[WhiteBlue]

do you understand that diesel engines have a huge surplus of air especially at low speeds, necessary to get combustion of the fuel ?
(so increased massflow/power by turbocharging (relatively) reduces the disproportionate mechanical losses of the NA diesel)
do you understand that turbocharging a diesel does not require a reduction in compression ratio ?(detonation is needed)
unlike turbocharging a SI engine, where lowering the CR is required to avoid detonation (in a fair comparison)
these are reasons why turbocharging a diesel is always advantageous
they do not apply to the SI engine

Do not worry about me not understanding your thoughts. The point is that the perceived disadvantages of turbocharged SI engines are going to disappear with the development of the technology. For instance compression ratio will increase in the years to come as it has already done in the past. The advantages of turbocharging particularly the use of waste energy to run higher operating pressures will stay.

[riff_raff]

....do you understand that turbocharging a diesel does not require a reduction in compression ratio ?(detonation is needed) unlike turbocharging a SI engine, where lowering the CR is required to avoid detonation (in a fair comparison)....these are reasons why turbocharging a diesel is always advantageous
they do not apply to the SI engine

Tommy Cookers-

While it is true that when boosting a CI it is not absolutely necessary to reduce the CR in order to prevent detonation like that occurring in an SI engine. But most highly boosted automotive and truck CI engines do use a greatly reduced CR, as well as lots of charge air cooling, and this is to reduce NOx emissions. While this reduced CR and significant charge air cooling actually hurts BTE, it's the best way to limit NOx emissions.

[pgfpro]

Some numbers I came up with using my spreadsheet pertaining to one of my favorite books I own
"McLaren Honda Turbo A Technical Appraisal" by Ian Bamsey

The RA168E (F.C.) settings
At 12000rpm it made 456KW at a BSFC of 272g/KW*h
This was at 2.5bar, 70*C IAT, 80*C FT and 0.98 excess air ratio with M.B.T or knock limit Ignition Timing

The fuel that was used was 84% Toluene, 16% n-Heptane with no Isooctane.
RON 101.8
MON 90.0
Density (at 15*C) 0.840
Net Calorific Value (Kcal/Kg) 9817
Stoichiometric Ratio 13.7
Reid vapor pressure (Kg/cm) 0.120
Initial Boiling Point (*C) 100.0
10% 105.0
50% 106.0
90% 108.0
End Point 116.0

Putting all these values into my engine spreadsheet I came up with 32% BTE. So as WB said earlier in one of his post with a 100KW added into the equation you could see a increase of 32% to 39% BTE I think the engineers would of love to have one these 24 years ago.

So what are these new engines going to be?

Changing the BSFC to 235g/KW*h I come up with .37% BTE engine only and .45 BTE with 100KW added.

Now that would be a game changer!!!

[olefud]

A similarly downsized NA engine is a bit on the wimpy side. In order to gain back enough power to be socially acceptable, the engine is subjected to turbocharging. But the NA engine would enjoy a cooler mixture, higher compression ratio (volumetric) and be even less weighty than the turbo. It’s all about power.

That's a funny way to see things. Needless to say that I don't agree. With the rest of the world I prefer more fuel efficient engines. But I see that isn't socially acceptable to you. I respect such personal choices.

Again, you’re missing the point. The NA engine is the more efficient engine for reasons including those mentioned. The social acceptability relates to the lower power of the NA engine and the inability to satisfy customers. Thus, at the price of lower efficiency, the engine is turbocharged solely to gain power and social acceptance on the showroom floor.

[WhiteBlue]

The social acceptability relates to the lower power of the NA engine and the inability to satisfy customers. Thus, at the price of lower efficiency, the engine is turbocharged solely to gain power and social acceptance on the showroom floor.

Your logic is somewhat confused. A car buyer who considers buying a car is not interested to know how much power his engine would hypothetically have without the turbo charger fitted. All he wants to know is the total power, the acceleration, the top speed, the fuel economy, the handling and looks of his object of desire. The chance that our guy will buy a downsized, turbocharged car with better fuel economy than an alternative NA engined option is rising all the time. I do not need to rely on my own experience to know that. Our expert from Garrett told us that in the interview that I posted in the opening post. Privately I'm just wondering how long people will listen to sales people who are bullshitting them with the story of wonderful efficiency of NA engines that unfortunately return only 15 miles to a gallon.

[Smokes]

Everyone here knows how turbocharging works.

in regards to the BTE of the engine

Assuming the the turbo compressor and impeller has spooled up to a working rpm to deliver an usable mass flow rate of air.

Turbocharging an engine will use the waste gasses to force compressed air into the engine. This statement is true right.

Does mean that the pumping losses of the engine are reduced and the that charge is mixed better due to swirl and increase compression ratio so that the energy generated on the power stroke is higher.

What is the pumping loss reduction and the typical useful work done by the fuel charge?

When the turbo compressor and impeller is either running to fast or to slow to deliver a mass flow rate.

Do the pumping losses increase to take into account the drag of the compressor and the manifold?

Also the valve overlap for a turbocharged is minimise compared to a N/A engine does this also reduce the charge mix?

how much useful work of the charge is lost when the engine cannot pull the ideal amount of air into the engine and the compression rate is dropped?

Does this also mean a turbocharged SI CI engine efficent operating range is dependant on the operating range of the turbo compressor and impeller.

[olefud]

Everyone here knows how turbocharging works.

in regards to the BTE of the engine

Assuming the the turbo compressor and impeller has spooled up to a working rpm to deliver an usable mass flow rate of air.

Turbocharging an engine will use the waste gasses to force compressed air into the engine. This statement is true right.

Does mean that the pumping losses of the engine are reduced and the that charge is mixed better due to swirl and increase compression ratio so that the energy generated on the power stroke is higher.

What is the pumping loss reduction and the typical useful work done by the fuel charge?

When the turbo compressor and impeller is either running to fast or to slow to deliver a mass flow rate.





Do the pumping losses increase to take into account the drag of the compressor and the manifold?

Also the valve overlap for a turbocharged is minimise compared to a N/A engine does this also reduce the charge mix?

how much useful work of the charge is lost when the engine cannot pull the ideal amount of air into the engine and the compression rate is dropped?

Does this also mean a turbocharged SI CI engine efficent operating range is dependant on the operating range of the turbo compressor and impeller.

Good questions. IMO, under full boost, not only would pumping losses be diminished, but some low level “compounding” work could be recovered with enough boost. This would depend on the high-boost duty cycle of the engine –high in a racing engine, low in a road engine.

In general, the valve overlap shouldn’t affect the charge mix in that the boost can be modulated to provide a full charge. However, the lack of overlap will affect the bit of free boost from pressure wave tuning of the exhaust.

Not sure of your “ideal charge” meaning. For a stoichiometric charge engine the “proper charge” is that having the fuel/air mixture in the amount necessary for the desired power level. Within the power range of the particular engine, it will be essentially the same for a NA or turbocharged otherwise similar engine.

As to the efficient operating range, IMO –others differ- with higher boost the combustion products don’t have enough expansion volume to extract the heat energy as work, i.e. there is still relatively high pressure as the power stroke terminates with the opening of the exhaust valve. A NA engine has a higher expansion volume relative to the quantity of combustion products

[Smokes]

Everyone seems to forget the basics of engine efficiency.

Strapping on a turbo just means more fuel and air can be drawn into the engine and compressed to a high enough ratio without knocking.

For the charge to start doing work it has to it has to over come engine friction pumping losses, power train inertia, some of the energy is turned in to heat and light. Also not all of the pressure generated by the charge is used up by the time the piston hits bdc.

All the turbo can does to improve efficiency is reduce pumping losses by using the waste energy from the exhaust gasses and force air into the cylinders.

More efficiency can be gained by reducing engine friction which is where illmor/mercedes were going before FIA banned using beryllium.

[olefud]

Everyone seems to forget the basics of engine efficiency.

Strapping on a turbo just means more fuel and air can be drawn into the engine and compressed to a high enough ratio without knocking.

For the charge to start doing work it has to it has to over come engine friction pumping losses, power train inertia, some of the energy is turned in to heat and light. Also not all of the pressure generated by the charge is used up by the time the piston hits bdc.

All the turbo can does to improve efficiency is reduce pumping losses by using the waste energy from the exhaust gasses and force air into the cylinders.

More efficiency can be gained by reducing engine friction which is where illmor/mercedes were going before FIA banned using beryllium.

You’re of course correct that efficiency can be improved in many ways, including reducing friction. But many of these have been exploited to the point of diminishing returns –and the object of this thread is turbo charging vis-à-vis efficiency. Further, the question of thermal efficiency differs greatly in competition and road engines –though the former changes towards the road engine with a fuel flow restriction.
IMO turbo charging in both cases is for purposes of increased power in SI engines in both cases. In the road engine case, the great majority of the operating time is spent outputting relatively low power. A small amount of air is allowed past the throttle and mixed with the appropriate volume of fuel to provide the low-power requirement. In otherwise identical turbo and NA engines and conditions, the fuel charge is essentially identical for the turbo and NA engine. Minor differences such as fuel mixture temp, compression ratio etc. tend to favor the NA engine slightly. This is true as long as the power requirement is at or below that for which the air pumping capacity of the NA engine satisfies the power requirement. Once the power demand exceeds this point, the NA engine needs a lower gear to pump more air while the turbo engine can provide the air at lower RPM by increasing the density (pressure) of the intake air. And of course, once the NA engine exceeds its maximum air pumping capacity, it can develop no additional power while the turbo engine, being in essence a two stage air pump, can provide greater quantities of air and greater power.
Since the both engines are essentially undifferentiated up to the max power of the NA engine, the efficiencies are very similar. However, since the advantage of the turbo engine is the ability to produce greater power –usually with diminished thermal efficiency than when operating in the domain in common with the NA engine- the advantage and purpose of turbo charging would seem to be power rather than thermal efficiency. Of course compounding is yet another subject.
But you’re right that there are many other parameters affecting both efficiency and power beyond turbo charging.