Turbo Vs N/A economy - off boost
Discussion
Extra economy might not actually be down to the engine as such. Because turbo engines produce more power and torque the manufacturers tend to run them with higher gearing.
Think that's why modern 'light pressure turbo' engines like Saab's Ecopower units can be more economical in gentle use than the normally aspirated versions.
Think that's why modern 'light pressure turbo' engines like Saab's Ecopower units can be more economical in gentle use than the normally aspirated versions.
ylee coyote said:
As we see lower CR gives you lower efficiency and lower specific outputs
..yes, but the OP was talking about off boost, not max power. Effective CR drops on any petrol engine when you throttle back.My hypothesis was that the lower CR of a turbocharged has the same effect as the restrictive throttle of a NA high CR engine when throttled back, ie both give the same low peak cylinder pressure.
Other than the effect of static CR on combustion space, I don't (yet) understand why getting compression ratio through piston geometry is any more efficient than getting it through using a turbocharger?
Boosted LS1 said:
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.
I guess what you are saying is that for a turbocharged car, as well as dropping the CR, you are effectively making the combustion space bigger;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.
Ultimately the combustion space is sized for the peak power output, so when you throttle back, the combustion space (and by this I'm also thinking about valve design / swirl etc.) is operating further from optimum; does this sound right? So its all about the physical geometry associated with dropping the CR, rather than the CR itself.
Mave said:
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?
I think the point is that the mass flow of air is greater with the lower CR to achieve the same output.Mave said:
Boosted LS1 said:
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.
I guess what you are saying is that for a turbocharged car, as well as dropping the CR, you are effectively making the combustion space bigger;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.
Ultimately the combustion space is sized for the peak power output, so when you throttle back, the combustion space (and by this I'm also thinking about valve design / swirl etc.) is operating further from optimum; does this sound right? So its all about the physical geometry associated with dropping the CR, rather than the CR itself.
Boosted.
Mave said:
[
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?
This is the point you are missing, if you have a lower thermal efficiency you need more air and fuel in the cylinder to provide the same BMEP.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?
Mave said:
ylee coyote said:
As we see lower CR gives you lower efficiency and lower specific outputs
..yes, but the OP was talking about off boost, not max power. Effective CR drops on any petrol engine when you throttle back.My hypothesis was that the lower CR of a turbocharged has the same effect as the restrictive throttle of a NA high CR engine when throttled back, ie both give the same low peak cylinder pressure.
Other than the effect of static CR on combustion space, I don't (yet) understand why getting compression ratio through piston geometry is any more efficient than getting it through using a turbocharger?
If you have a engine management system which detects early detonation then you can run with a conventional c/r then you can have the best of both worlds
partial throttle efficiency ,off boost response of a n/a and full power of forced induction ...two engines in one.....
ylee coyote said:
Mave said:
ylee coyote said:
As we see lower CR gives you lower efficiency and lower specific outputs
..yes, but the OP was talking about off boost, not max power. Effective CR drops on any petrol engine when you throttle back.My hypothesis was that the lower CR of a turbocharged has the same effect as the restrictive throttle of a NA high CR engine when throttled back, ie both give the same low peak cylinder pressure.
Other than the effect of static CR on combustion space, I don't (yet) understand why getting compression ratio through piston geometry is any more efficient than getting it through using a turbocharger?
If you have a engine management system which detects early detonation then you can run with a conventional c/r then you can have the best of both worlds
partial throttle efficiency ,off boost response of a n/a and full power of forced induction ...two engines in one.....
Ok, this is too broad a topic to generalise.
However sometimes it is neccessary to generalise a little in order to see trends.
First off MAVE, throttling does not lower your effective compression ratio. It does lower your mass flow and therefore your volumetric efficiency but it's confusing when you talk about lowering dynamic compression ratios via throttling.
Often changing cam timing and/or duration will change your dynamic compression ratio but that's another story. Thermodynamic efficiency of an engine has more to do with expansion ratio than effective compression ratio anyway.
Now I'm trying to understand what MAve is saying- this is my interpretation and my conclusion:
Ok, let us assume we have similar boxer six cylinder 3.6 litre engines,
-identical cam timing
-similar friction
-the same level of squish
-we're not knock limited as we're running at part load
one is running an 8:1 compression ratio
the other is running an 11.3:1 CR
For the purpioses of this theoretical exercise the boosted engines (the lower CR one) is running totally off boost.
The engines are asked to perform a task and a required load is required off it- say 20 Nm of torque.
To achieve this 20 Nm of torque the NA engine will achieve this "easier" under non boost conditions so will be MORE THROTTLED-the throttle will be more closed. The Boosted engine will have to have the throttle more open to achieve this same target torque.
This is because a higher compression ratio boosted BMEP or torque per litre (no boost-rememeber).
Running an engine less throttled will reduce pumping losses .
However- the combustion efficiency gain increase efficiency much more than the lowering of pumping losses. You can't even compare.
In the above example the CR effect alone on fuel economy going by rudimentary thermodynamic gas law equations will give about 9% benefit.
You only have to do a gas exchange analysis and consider an engine Pressure-Volume diagram/trace and deduct the slight pumping loss benefit you could gain and compare.
Someone else perhaps raised a more relavant point, by talking about taller gearing of the vehicle.
The other point worth mentioning that makes this hypothetical argument largeley accademic is that very few manufacturers would chose to keep the same Cam timing and duration when optimising their Turbo charged engine compared to their NA engine- and this has a larger effect to pumping losses, effective compression ratio and even burn in extreme cases.
Last point is that the burn rate will be effected by the compression ratio- as the laminar flame velocity will increase and the residual gas content left within the cylinder will go down (both favouring the NA high CR engine)- this has not been considered on my off-the-cuff "9%"
However sometimes it is neccessary to generalise a little in order to see trends.
First off MAVE, throttling does not lower your effective compression ratio. It does lower your mass flow and therefore your volumetric efficiency but it's confusing when you talk about lowering dynamic compression ratios via throttling.
Often changing cam timing and/or duration will change your dynamic compression ratio but that's another story. Thermodynamic efficiency of an engine has more to do with expansion ratio than effective compression ratio anyway.
Now I'm trying to understand what MAve is saying- this is my interpretation and my conclusion:
Ok, let us assume we have similar boxer six cylinder 3.6 litre engines,
-identical cam timing
-similar friction
-the same level of squish
-we're not knock limited as we're running at part load
one is running an 8:1 compression ratio
the other is running an 11.3:1 CR
For the purpioses of this theoretical exercise the boosted engines (the lower CR one) is running totally off boost.
The engines are asked to perform a task and a required load is required off it- say 20 Nm of torque.
To achieve this 20 Nm of torque the NA engine will achieve this "easier" under non boost conditions so will be MORE THROTTLED-the throttle will be more closed. The Boosted engine will have to have the throttle more open to achieve this same target torque.
This is because a higher compression ratio boosted BMEP or torque per litre (no boost-rememeber).
Running an engine less throttled will reduce pumping losses .
However- the combustion efficiency gain increase efficiency much more than the lowering of pumping losses. You can't even compare.
In the above example the CR effect alone on fuel economy going by rudimentary thermodynamic gas law equations will give about 9% benefit.
You only have to do a gas exchange analysis and consider an engine Pressure-Volume diagram/trace and deduct the slight pumping loss benefit you could gain and compare.
Someone else perhaps raised a more relavant point, by talking about taller gearing of the vehicle.
The other point worth mentioning that makes this hypothetical argument largeley accademic is that very few manufacturers would chose to keep the same Cam timing and duration when optimising their Turbo charged engine compared to their NA engine- and this has a larger effect to pumping losses, effective compression ratio and even burn in extreme cases.
Last point is that the burn rate will be effected by the compression ratio- as the laminar flame velocity will increase and the residual gas content left within the cylinder will go down (both favouring the NA high CR engine)- this has not been considered on my off-the-cuff "9%"
Edited by Marquis_Rex on Tuesday 29th May 14:07
Marquis_Rex said:
First off MAVE, throttling does not lower your effective compression ratio. It does lower your mass flow and therefore your volumetric efficiency but it's confusing when you talk about lowering dynamic compression ratios via throttling.
OK, got to read all of the post to fully digest, but I'm surprised by this comment. Surely when you look at the PV diagram, you need to consider the air all the way from inlet to TDC, not just from the manifold to TDC?Edited by Marquis_Rex on Tuesday 29th May 14:07
To get less mass flow into the cylinder, the pressure downstream of the throttle [b] must [b] be less; there is a single relationship between pressure at inlet manifold and mass flow at a given engine speed.
Quick question; do you agree that BMEP varies with throttle setting?
Marquis_Rex said:
Last point is that the burn rate will be effected by the compression ratio- as the laminar flame velocity will increase and the residual gas content left within the cylinder will go down (both favouring the NA high CR engine)- this has not been considered on my off-the-cuff "9%"
I must admit, I'm a total numpty re combustion proces.Edited by Marquis_Rex on Tuesday 29th May 14:07
I sthis improvement in combustion efficiency due to combustion shape / volume, or due to absolute pressure?
ylee coyote said:
Mave said:
ylee coyote said:
As we see lower CR gives you lower efficiency and lower specific outputs
..yes, but the OP was talking about off boost, not max power. Effective CR drops on any petrol engine when you throttle back.My hypothesis was that the lower CR of a turbocharged has the same effect as the restrictive throttle of a NA high CR engine when throttled back, ie both give the same low peak cylinder pressure.
Other than the effect of static CR on combustion space, I don't (yet) understand why getting compression ratio through piston geometry is any more efficient than getting it through using a turbocharger?
If you have a engine management system which detects early detonation then you can run with a conventional c/r then you can have the best of both worlds
partial throttle efficiency ,off boost response of a n/a and full power of forced induction ...two engines in one.....
My understanding of thermodynamics is that you need to consider the pressure all the way from inlet to TDC. So, although you get a CR of, say, 10 when operating at WOT, it is only 2 or 3 when operating at idle.
Ignoring the effect of BMEP on combustion efficiency (may be a poor assumption I know!) I think that there is a fixed torque for a particular BMEP at a particular engine speed, assuming fuel air ratios are kept under control. the only way to vary BMEP is to effectively dump pressure at the throttle. This drops the compression ratio from inlet to TDC.
Mr2Mike said:
Mave said:
[
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?
This is the point you are missing, if you have a lower thermal efficiency you need more air and fuel in the cylinder to provide the same BMEP.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?
Matthew-TMM said:
Mave said:
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?
I think the point is that the mass flow of air is greater with the lower CR to achieve the same output.PS, I recognise that I'm sat on the other side of the fence from a few of you guys, honestly not trolling, just trying to improve my understanding! My background is in gas turbine design, and unfortunately some aspects of off-design performance read across, but some do not! (EG combustion efficiency in a GT is pretty constant at more than 10% power, you just bung in a single assumption of ~95%!!)
Mave said:
Mr2Mike said:
Mave said:
[
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?
This is the point you are missing, if you have a lower thermal efficiency you need more air and fuel in the cylinder to provide the same BMEP.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?
Lower CR = Lower Thermal efficiency
Lower Thermal efficiency = less work extracted from a set amount of fuel and air
To get the same power as high CR engine, requires more fuel and air to be burnt, which means opening the throttle wider.
Brake Specific Fuel Consumption of turbo charged engine is most always higher than that of an normaly aspirated engine.
Mr2Mike said:
What is "Combustion efficiency"? If this is what you interpret as the ability to exatct useful work from the fuel, then this is actually thermal efficiency which is reduced as the CR is lowered.
In my terms, combustion efficiency = energy added to the air during the combustion cycle / energy available in the fuel. The things which effect this are how well the fuel mixes, shaped of the combustion space etc. I would understand thermal efficiency as the efficiency of the overall cycle, ie the one which can never exceed 50% ish for a sensible CR.There are lots of things other than CR which make up the efficiency of an engine; combustion efficiency being one of them (and I admit I don't know how significant that it).
What I was trying to do was get back to the issue of the primary effect of CR on efficiency (ie the thermal efficiency), not be sidelined by the secondary effects (eg dropping the CR may have a detrimental effect on combustion efficiency due to a change in shape of the piston bowl)
Mr2Mike said:
Lower CR = Lower Thermal efficiency
Agreed. But a high CR engine does not achieve that high CR at part power.Imagine a p-v diagram for an N/A engine with an 8:1 compression ratio for instance, at WOT, the area inside gives the work (as you will know anyway). Then imagine the same engine with a 12:1 compression ratio at WOT and it's p-v diagram which should show a grater area and hence more work done. To reduce the output of the higher CR engine to that of the low CR engine you will need to throttle the engine (ignoring everything else you could do of course), in order to reduce the area in the trace to that of the low CR engine at WOT. When you've throttled the high CR engine to this point, the power output of both will be the same, but the mass flow of air and hence also fuel into the high CR engine will be less.
That's how I see it anyway.
That's how I see it anyway.
Probably not helping in the debate but you can have a look at it from the other side: audi with their turbo-ed quatro were the first to master turbo with a higher compression ratio to get best of both worlds (first applied in their rallying stuff). The rest tries serial turboing (two turbo's: one small to make up the loss during the revs that the big one is not on boost). Only saab uses a very small turbo to get engine efficiency up (is a more economical&clean engine for same output and size as NA). From this it could prove to you that higher compression compared to turbo engine off boost, the NA wins. Major remark is that you sort of need to compare the same engine, which is very difficult.....
Let the debate continue.
Rob
Let the debate continue.
Rob
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