Turbo Vs N/A economy - off boost

[_Al_]

I've just been told that the lower compression ratio of a turbo engine increases its' off-boost fuel economy above that of a similar naturally aspirated engine.

I suspect that this isn't correct, does anyone know enough of the science to give me a definitive answer?

[Boosted LS1]

_Al_ said:

I've just been told that the lower compression ratio of a turbo engine increases its' off-boost fuel economy above that of a similar naturally aspirated engine.

I suspect that this isn't correct, does anyone know enough of the science to give me a definitive answer?

Other way around imo. I'm assuming the n/a engine has a higher c/r but is otherwise the same engine? Lower c/r makes the engine less efficient off boost. You could of course boost an engine on stock c/r and then you'd be very efficient and fast Or you could optimise a low c/r engine to make it as efficient as a n/a engine.

Boosted.

[Mave]

I think that in the end there should be very little difference.
In a petrol engined car, you deliberately drop the effective C/R by closing the throttle. With a turbocharged car, you just don't close the throttle as much for a given power demand...

[cptsideways]

A lower compression ratio will always be less efficient, the throttle opening only dictates the volume of air into the cylinder so to get the same effective CR the turbo car will require more air for the same CC. More air needs more fuel.

However dropping the octane off boost in theory would mean you can advance the ignition further to increase the efficiency, but that wont work on boost.

[Mave]

cptsideways said:

A lower compression ratio will always be less efficient, the throttle opening only dictates the volume of air into the cylinder so to get the same effective CR the turbo car will require more air for the same CC. More air needs more fuel.

However dropping the octane off boost in theory would mean you can advance the ignition further to increase the efficiency, but that wont work on boost.

Doesn't the throttle opening also drop the pressure at the inlet manifold? So you get the same CR from inlet manifold to max compression, but the CR from atmosphere to max compression (which is what dictates efficiency) drops....

[GreenV8S]

Mave said:

cptsideways said:

A lower compression ratio will always be less efficient, the throttle opening only dictates the volume of air into the cylinder so to get the same effective CR the turbo car will require more air for the same CC. More air needs more fuel.

However dropping the octane off boost in theory would mean you can advance the ignition further to increase the efficiency, but that wont work on boost.

Doesn't the throttle opening also drop the pressure at the inlet manifold? So you get the same CR from inlet manifold to max compression, but the CR from atmosphere to max compression (which is what dictates efficiency) drops....

For a given displacement, the engine with the lower CR has the greater volume at TDC. This means it requires more charge mass to reach a given pressure or will produce less pressure for a given charge mass. Hence it will burn slightly less efficiently for a given power output.

[Boosted LS1]

Mave said:

cptsideways said:

A lower compression ratio will always be less efficient, the throttle opening only dictates the volume of air into the cylinder so to get the same effective CR the turbo car will require more air for the same CC. More air needs more fuel.

However dropping the octane off boost in theory would mean you can advance the ignition further to increase the efficiency, but that wont work on boost.

Doesn't the throttle opening also drop the pressure at the inlet manifold? So you get the same CR from inlet manifold to max compression, but the CR from atmosphere to max compression (which is what dictates efficiency) drops....

I can't understand what you mean but if it helps, engines run in a vacuum state. High vacuum at idle but opening the throttle reduces the vacuum but they won't ever go to positive without forced induction.

Boosted.

[Mave]

Boosted LS1 said:

I can't understand what you mean but if it helps, engines run in a vacuum state. High vacuum at idle but opening the throttle reduces the vacuum but they won't ever go to positive without forced induction.

Boosted.

OK, let me try to explain my mad reasoning a different way;

The compression ratio we are interested in for determining efficiency is from atmosphere to TDC, not inlet manifold to TDC, as the engine is doing work on the air all the way from the air intake.

(If we were only interested in inlet manifold to TDC then we would find that the efficiency was fairly constant at all throttle openings, which is not the case.)

The flow through an engine is set only by inlet manifold pressure, as the geometry of the downstream passageways and pistons etc is fixed. So, power and efficiency are independent of how you get that manifold pressure.

Comparing the high CR NA with a low CR turbocharged car,
For a given idle power, you are trying to achieve a flow and given total CR from atmosphere to TDC.

In the high CR NA car, the throttle creates a very low manifold pressure, to keep a low total CR and hence low idle power.

In the low CR turbocharged car, the throttle needs to create less resistance as you need slightly less manifold vacuum to give the same total CR (and hence same power).

Ultimately though, you have the same CR and same flow rate for both engines.

Does this make sense? I was thinking of drawing up what I think is happening to the pressures, but haven't done it yet...

[Mave]

GreenV8S said:

Mave said:

cptsideways said:

A lower compression ratio will always be less efficient, the throttle opening only dictates the volume of air into the cylinder so to get the same effective CR the turbo car will require more air for the same CC. More air needs more fuel.

However dropping the octane off boost in theory would mean you can advance the ignition further to increase the efficiency, but that wont work on boost.

Doesn't the throttle opening also drop the pressure at the inlet manifold? So you get the same CR from inlet manifold to max compression, but the CR from atmosphere to max compression (which is what dictates efficiency) drops....

For a given displacement, the engine with the lower CR has the greater volume at TDC. This means it requires more charge mass to reach a given pressure or will produce less pressure for a given charge mass. Hence it will burn slightly less efficiently for a given power output.

Ah, but you don't throttle the engine back so much, so you get a slightly higher manifold pressure (or slightly less vaccuum), which offsets the lower CR.

[Boosted LS1]

Mave said:

Boosted LS1 said:

I can't understand what you mean but if it helps, engines run in a vacuum state. High vacuum at idle but opening the throttle reduces the vacuum but they won't ever go to positive without forced induction.

Boosted.

.

Comparing the high CR NA with a low CR turbocharged car,
For a given idle power, you are trying to achieve a flow and given total CR from atmosphere to TDC.

In the high CR NA car, the throttle creates a very low manifold pressure, to keep a low total CR and hence low idle power.

In the low CR turbocharged car, the throttle needs to create less resistance as you need slightly less manifold vacuum to give the same total CR (and hence same power).

Ultimately though, you have the same CR and same flow rate for both engines.

Does this make sense?

I understand what you mean. You're basically saying it's about supply and demand? However, this doesn't equate to equal efficiency imo. Fuel burns less efficiently in a big volume chamber and there will be wastage. In a small volume chamber you get a faster burn due to the denser charge. The c/r won't be the same either given the differing cylinder volumes.

Boosted.

[cptsideways]

Assuming identical engines, same rpm just one low one high CR:

The cranking action can only draw in so much air, each will draw the same amount. However one gets squished more than the other, same volume of air drawn in but the higher CR one will output more energy in the combustion process, in basic terms.

[Mr2Mike]

Lower compression = lower efficiency, this has always been one of the downsides of forced induction. Saabs variable compression ratio system would have really helped to perk up the off-boost reponse and economy of a turbocharged engine, but I believe it's been shelved for now.

[Mave]

Boosted LS1 said:

Mave said:

Boosted LS1 said:

I can't understand what you mean but if it helps, engines run in a vacuum state. High vacuum at idle but opening the throttle reduces the vacuum but they won't ever go to positive without forced induction.

Boosted.

.

Comparing the high CR NA with a low CR turbocharged car,
For a given idle power, you are trying to achieve a flow and given total CR from atmosphere to TDC.

In the high CR NA car, the throttle creates a very low manifold pressure, to keep a low total CR and hence low idle power.

In the low CR turbocharged car, the throttle needs to create less resistance as you need slightly less manifold vacuum to give the same total CR (and hence same power).

Ultimately though, you have the same CR and same flow rate for both engines.

Does this make sense?

I understand what you mean. You're basically saying it's about supply and demand? However, this doesn't equate to equal efficiency imo. Fuel burns less efficiently in a big volume chamber and there will be wastage. In a small volume chamber you get a faster burn due to the denser charge. The c/r won't be the same either given the differing cylinder volumes.

Boosted.

OK, see what you're saying, so there is a difference in the combustion physics determined by the different chamber shapes at TDC? I was just thinking about it from a pure pressure / temperature viewpoint.

[Mave]

cptsideways said:

Assuming identical engines, same rpm just one low one high CR:

The cranking action can only draw in so much air, each will draw the same amount.

No they won't, to get the same power the low CR engine will have less throttling effect at the butterfly, so the higher manifold pressure will allow more air into cylinder. If low CR always equalled low cylinder charge, then turbocharging wouldn't work....

[Mave]

Mr2Mike said:

Lower compression = lower efficiency, this has always been one of the downsides of forced induction.

But why? what is causing this reduced efficiency? The whole point I was trying to make is that the effective CR of a turbocharged engine making a given power is the same as a non-turbocharged engine.

Compression Ratio is just a convenient way of relating mechanical geometry to peak cyclinder pressure for a NA car. It just gets more complex with a turbo because some of the compression is done before you get into the piston.

[Mr2Mike]

Mave said:

Mr2Mike said:

Lower compression = lower efficiency, this has always been one of the downsides of forced induction.

But why? what is causing this reduced efficiency?

Quite simply the thermal efficiency of the otto cycle is related to compression ratio. The more you squeeze the mixture before igniting it, the more power you get out of it.

Mave said:

The whole point I was trying to make is that the effective CR of a turbocharged engine making a given power is the same as a non-turbocharged engine.

Assuming no other losses then yes. In practice it won't be as a turbocharged 4 stroke engine is usually less efficient than a naturally aspirated engine due to increased pumping losses and higher inlet temperatures etc.

This also presupposes the turbo is actually doing something. A turbo doesn't do very much when you are cruising at constant speed, hence you fall back to the compression determined by the static CR and throttle opening. More throttle opening will be required in this case to achieve the same CR as naturally aspirated engine with a higher static CR, which means higher fuel consumption.

Edited by Mr2Mike on Monday 28th May 11:33

[GreenV8S]

Mave said:

GreenV8S said:

For a given displacement, the engine with the lower CR has the greater volume at TDC. This means it requires more charge mass to reach a given pressure or will produce less pressure for a given charge mass. Hence it will burn slightly less efficiently for a given power output.

Ah, but you don't throttle the engine back so much, so you get a slightly higher manifold pressure (or slightly less vaccuum), which offsets the lower CR.

I think I see what you mean.

From the point of view of combustion efficiency, the NA engine with a higher static CR should be more efficient. But this engine will also have more manifold depression for a given power, which will mean the pumping losses would be higher.

The way I look at it, the increased manifold depression is a consequence of the improved combustion efficiency - for a given power output the higher CR engine will require less charge mass and hence more manifold depression. This means that it will incur higher pumping losses, but pumping losses are only a small part of the overall losses and gut feeling is that they would be very small compared to the gains from the increased combustion efficiency.

[ylee coyote]

As we see lower CR gives you lower efficiency and lower specific outputs
However if you run sophisticated engine management (Like the SAAB Trionic) you can run normal C/R's (9.3 to 1) and get the best of both worlds
However with normal engine management systems you are stuck with the compromise

I get 28-32 mpg out of my turbo monster when not boosting
(7 when I am )

[Mave]

Mr2Mike said:

Mave said:

Mr2Mike said:

Lower compression = lower efficiency, this has always been one of the downsides of forced induction.

But why? what is causing this reduced efficiency?

Quite simply the thermal efficiency of the otto cycle is related to compression ratio. The more you squeeze the mixture before igniting it, the more power you get out of it.

Mave said:

The whole point I was trying to make is that the effective CR of a turbocharged engine making a given power is the same as a non-turbocharged engine.

Assuming no other losses then yes. In practice it won't be as a turbocharged 4 stroke engine is usually less efficient than a naturally aspirated engine due to increased pumping losses and higher inlet temperatures etc.

This also presupposes the turbo is actually doing something. A turbo doesn't do very much when you are cruising at constant speed, hence you fall back to the compression determined by the static CR and throttle opening. More throttle opening will be required in this case to achieve the same CR as naturally aspirated engine with a higher static CR, which means higher fuel consumption.

Edited by Mr2Mike on Monday 28th May 11:33

OK, if you're happy with PV / TS diagrams, I'll try to sketch out what I'm on about. I see what you mean about some increased pumping losses (I assume you are talking about inefficiencies in the compressor wheel of the turbo), but with regards to having a higher throttle opening to achieve the same CR, why does this burn more fuel? You've got the same mass flow of air, just at a slightly higher pressure, so you need the same amount of fuel? The temperature rise associated with a turbo doesn't really matter that much; either you get the temperature rise by compressing the air in the compressor wheel of the turbo, or you get it compressing it in the cylinder! If anything, the turbo should be slightly better because you can intercool it which lowers the charge temperature for a given peak cylinder pressure.

[Boosted LS1]

Mave,

Something else to consider is that charge density is increased with a boosted application. So a low c/r engine has a greater volume chamber into which we can pack a lot more fuel molecules for a longer burn, increasing the length of the power stroke gaining a torque increase over a n/a engine. The denser the molecules the better the burn, this applies to atmo engines as well. Inlet valve timing is a critical event as is chamber design, squish and quench. Small diameter chambers work well so do small bore engines as the flame front hasn't got so far to travel in order to complete the burn.

Boosted.