Turbocahrgers and efficiency

thefullcollapse

thefullcollapse

thefullcollapse

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ive been writing an essay on the efficiency of internal combustion engines recently and decided to add a sectiona bout turbochargers.
everyone i speak to either isnt sure or says that turbochargers boost efficiency.

i dont see how this works tbh, wonderd if someone could shed some light please
 
Engines have a volumetric efficiency. A portion of their power is required to "suck" air in through the ports. You can improve the volumetric efficiency by using forced induction because the supercharger/turbocharger pumps the air in. In the case of a turbocharger it uses waste gas to drive it so doesn't rob any further power from the engine so it improves efficiency that way.
 
The power gained from increased air density is greater than the power lost to the turbine, so you get a positive feedback in the system that continues until the turbine reaches maximum flow, or in practice when a valve opens to maintain a certain pressure.
 
volumetric efficiency will be improved as when the air is compressed you can fill the cylinder with more than its volume of air, taking volumetric efficiency over 100%

the power sapped from a turbo or blower, even ancillaries such as alternator, air con etc will reduce the mechanical efficiency as some of the power produced is used to run them instead of passing the power through the drivetrain
 
As nitro junkie says, I'd have thought that a 4 pot turbo on boost would be slightly less fuel efficient than an equivalent power NA 6 pot. Though its off boost cruising efficiency would be higher than a 6 pot of larger cc's.
 
Yup and just to back that up there is an optimum amount of boost to run for optimum efficiency. Run less and the engine will be wasting power driving the blower, run too much and the engine will be producing much more power but using more fuel per hp as a result.
 
Turbo chargers increase volumetric efficiency, i.e. they stuff more air into the engine.

A turbocharged petrol engine will typically have lower thermal efficiency than a normaly aspirated engine, i.e. they use more fuel to achieve the same power output. This partly due to increases pumping losses, i.e. the engine has to work harder to push the exhaust gases out, the lower compression ratio required to eliminate detonation and the wasted heat that the turbo puts into the inlet charge.
 
the vast majority of the energy the the turbine produces comes from the under expanded exhaust gas the back pressure cause is of relatively little consequence. the efficiency of turbo engines can be considerably higher than na engines but it requires lower boost levels so the the afr can remain for closer to stoic. the efficiency of a engine is in the simplistic model only really due to compression ratio, with a turbo you can reach a higher dynamic cr before knock which is where the extra efficiency comes from.
 
cossey3 said:
the vast majority of the energy the the turbine produces comes from the under expanded exhaust gas the back pressure cause is of relatively little consequence.
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The TIP (Turbine inlet pressure) of a typical OEM turbo install can be twice that of the boost pressure. Hardly 'little' consequence.

They will more efficient at higher load where the pressure reduces pumping losses and the dynamic compression ratio is higher of that of a NA at that load. The beauty of the turbo is that not all compression takes place in the cylinder so you can control the temperature of the intake charge better with an intercooler.
 
With CI engines (diesel), a turbocharger can increase the efficiecy because you can have a higher compression ratio without knocking. A higher compression ratio always mean more efficiency, but with SI (petrol) engines, you get knocking problems with a high compression ratio.


A turbocharger reduce the volumetric efficiency but as I understand the reduction of volumetric efficiency does not reduce the fuel efficiency because you simply burn less fuel with less air. A turbo-charger covert the normally wasted heat into mechanical power to drive the compressor, that is also why a turbo charger is more efficeincy than a super charger because a super charger runs directly off the crank.
 
turbos increase volumetric efficiency, not reduce is - they put more than the volume of the cylinder into the cylinder by compressing the air

regarding knock etc; hence why lower compression heads/pistons are used with a turbo on a SI engine. as you say CI engines are not prone to knock so can run around 25-30:1 compression compared to 10-11:1 maximum with a SI engine
 
Depends if you base VE on actual displacement or effective displacement.

Ie a 2 litre running 1 bar of boost increases VE if you consider it as a 2 litre, but will probably reduce VE if considered as a 4.0 litre.

VE is only really related to *** efficiency also, efficiency in my book is more about the engine fuel consumption at typical road conditions.
 
thats a fair point, I take it based upon original displacement

efficiency can be measured in many things depending what you want from it
volumetric efficiency
fuel efficiency
fuel conversion efficiency
mechanical efficiency
thermal efficiency

fuel efficiency may go down when volumetric efficiency is increased but actual fuel conversion efficiency will go up (if other factors are set up correctly)

from these various mean effective pressures can be calculated which are more useful when comparing engines against one another
 
There's a lot of threads in this subforum where I sit back and think, how do people know this sort of stuff. I read them even though it's of no concern to me.

This thread is one of them :confused:
 
Nitro_Junkie said:
turbos increase volumetric efficiency, not reduce is - they put more than the volume of the cylinder into the cylinder by compressing the air

regarding knock etc; hence why lower compression heads/pistons are used with a turbo on a SI engine. as you say CI engines are not prone to knock so can run around 25-30:1 compression compared to 10-11:1 maximum with a SI engine
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CI engine rely on knock to burn :D


With turbo engines, Compression will reduce economy however with newer engines boost pressures are fairly low and cmpression ratio isn't reduced that much like the old school performance engines. A turbocharger will help reduce pumping losses and due to better VE (volumetric efficiency), even at part load, the dynamic compression ratio is better.
Properly specced turbos reduce pumping losses, even under part throtte, as air is being 'helped' into the engine. A part load the amount of airflowing through the engine is fairly small, such that the turbo doesn't really offer much resistance to the flow.

On the induction stroke the pistons are working against a vacuum and this vacuum is trying to slow the pistons down. With a turbo at part throttle the compressor is reducing this vacuum (not creating boost though) this means less energy is lost by pistons due to this reduced pressure difference.

Also, on the exhaust side, the gases are hot and trying to expand, they want to leave the engine and this pressure difference, even with the turbo back pressure at part load will always be helping the gases leave the cylinders. A lot of the gas will escape without the help of the piston pushing them out.

The lack of pumping losses in a diesel also helps their economy (no throttle) and BMWs valvetronic system that varies the inlet valve lift negating the need for a throttle plate also helps improve economy, again by reducing pumping losses.

Downsizing is where this all comes from, OEMs now using smaller turbo engine to provide the same output as larger NA engines, this allows better emission figures.
Comparing two engines of the same output, one with a turbo+less cylinders and one without a turbo, friction is a big factor as well as complexity and where the engine is in it's power band. Future hybrids with very complex controls systems are likely to have engines creating more power than the car needs with the excess being stored in the batteries. This will mean more of the energy from the fuel being burnt will be used by the vehicle.

Direct injection engines also help with economy and turbo response. There is no fuel in the inlet system, this allows all the available flow to be used by air, again increasing the VE of the engine.


Grabbed most of that from another thread I posted in on another forum but of the same subject
 
5tephen said:
There's a lot of threads in this subforum where I sit back and think, how do people know this sort of stuff. I read them even though it's of no concern to me.

This thread is one of them :confused:
Click to expand...

an obsession with cars, and a degree in progress for Performance Car Technology :)
 
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