Could somebody please explain the effect of turbo power on a car engine. I understand that it adds power to the engine but to what degree?
Specifically, what would be the difference between a Mercedes 1.8 petrol engine and the 1.8 Kompressor version (which I understand is MB's terminology for 'turbo') powering the same vehicle?
I imagine that the assisted version is thirstier but, again, to what degree?
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Mercedes Kompressor engines are supercharged.
A supercharger compresses the inlet air charge, and is driven by the crankshaft (typically via a belt)
A turbocharger compresses the inlet air charge and is driven by the exhaust gases passing through a turbine.
Turbochargers allow more air to be forced into the cylinders, which can be mixed with more fuel, to create a bigger bang, and more power.
How much more power (and how much more fuel is used) depends on the size of the turbocharger, and the general state of tune of the engine. And as always, how you drive it. Turbo engines, driven gently, are typically not much thirstier than a naturally aspirated engine of equivalent size, but because the turbo allows them to flow more air (and therefore more fuel) in hard use conditions, they have the capability to be significantly thirstier when driven hard.
My 1.8T Golf GTI will do a genuine 40 mpg on a gentle motorway run, equal I suspect to the non-turbo version. Squirting it round the lanes, using full throttle, you can get it down into the teens.
Last edited by: DP on Wed 6 Jul 11 at 21:46
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The limiting factor for cars regarding power is how much air you can get into the engine in a given period of time (increasing the fuel is the easy bit):- either you need a bigger engine, run the engine at higher revs, or compress the air going in (or all 3!).
Turbos are basically exhaust-driven fans which compress the air going into the engine (superchargers which I think were in previous Kompressor Mercs are not exhaust driven, but are fans driven directly by a belt from the crankshaft, similar to an alternator or aircon pump) - for example if you squeeze air into the engine at twice atmospheric pressure you can add twice the amount of fuel, and thus release twice the power of air simply being drawn in at atmospheric pressure.
(keeping it simples.. I know it's not quite atmospheric).
In the old days the main benefit was simply more power from a given size of engine with little added weight, and with a degree of fuel saving over using a bigger engine as the fan is driven by 'waste' energy - the exhaust gases. The downside was potentially sluggish response before a sudden rush of power, and increased complexity/cost to manufacture/maintain.
The explosion of petrol-turbo units (sales, not literally!) is because a high-geared small turbo unit is more efficient than a larger non-turbo, or a similar sized engine running at higher RPM/lower gearing (see VTEC) - basically manufacturers are under enormous pressure to reduce CO2 output and see turbos as the answer.
Summary: turbos add power (from ~15-50% or more depending upon state of tune - in the old days this was subject to poor response at low revs while the turbo spun up to speed), but more importantly today produce higher torque throughout the rev range enabling a small unit to drive much like a large muti-cylinder engine - with a substantial fuel saving unless outright performance is the aim.
EDIT: pretty sure the very latest Merc Kompressors are turbos rather than s/chargers.
Last edited by: Lygonos on Wed 6 Jul 11 at 21:56
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It's a relatively small turbo on that engine, as above it'll make it drive almost comparably with a 2.5 litre traditional engine, so it should be no slouch.
The only bit that some manufacturers are still struggling with is boost threshold (I'm looking at you astra vxr), which can be tiresome moving about in a car park for example. The turbo engine has lower compression ratio and feels a bit gutless off boost compared to the non turbo equiv.
Some (most?) manufacturers have got this well and truly nailed these days though.
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>> The turbo engine has lower compression ratio and feels a bit gutless off boost compared to the non turbo equiv.
This effect is more marked the bigger the turbo is. A good example was the late 90s Golf GT TDI 150 - at the time they sold the engine in 110, 130 and 150bhp versions, with the higher powered models having a narrower useable power band. The 150 was at the limit of driveability, but engine designs have improved greatly over the last 10 years to allow a better spread of power from the same arrangement.
Some manufactures fit two turbos - a small one that spins up quickly but doesn't supply much boost, and a big one that takes longer to spin up but offers a much greater whack of power. The BMW 535d is a good example of this, as with the right computer control to knit the two turbos' effects together smoothly there is a useful power boost at all points in the rev range with very little lag.
Another method of achieving the same effect is to use a variable vane turbocharger (see smaller BMWs, my Mondeo) where the angle of attack of the vanes increases at higher revs. This allows the turbo to spin up more quickly like a small one, then deliver greater boost like a big one. More to go wrong though, obviously.
Last edited by: Dave_TDCi on Wed 6 Jul 11 at 23:34
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Many thanks for all contributions.
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>> Another method of achieving the same effect is to use a variable vane turbocharger (see
>> smaller BMWs, my Mondeo) where the angle of attack of the vanes increases at higher
>> revs. This allows the turbo to spin up more quickly like a small one, then
>> deliver greater boost like a big one. More to go wrong though, obviously.
Also used in the Golf GT TDI 130/150 you mention. It does work remarkably well (the turbo in my 130 is boosting at 1400 RPM and delivering peak torque 500 RPM later), but the mechanism can gunk up. If it sticks in low rev / high angle of attack mode, this triggers an overboost condition under hard acceleration, and limp-home mode. Worse still, a lot of garages won't strip and clean the turbo (all it needs), instead opting to replace them at a cost of many hundreds of pounds. Mine has been fine (touch wood), but this is one of the few known / common faults with the PD engine.
It's a good point though that the size of turbo, boost level and state of tune of the engine make all turbo experiences very different. The light pressure turbo in my old S60 2.0T was so subtle in operation it simply felt like a bigger capacity naturally aspirated engine. A relatively high compression ratio and small, responsive turbo virtually eliminated lag, and the engine felt perky under pretty much any condition. An old 'big turbo' Escort Cosworth by contrast could barely pull it's own weight below 2500 RPM, and then by 3k it was like being rear ended by an artic. :-) The price of an engine that was designed to be tweaked to >500 bhp in competition use on largely standard components.
Two very different approaches to turbocharging, with two very different goals, and outcomes.
Last edited by: DP on Fri 8 Jul 11 at 11:59
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>> I imagine that the assisted version is thirstier but, again, to what degree?
>>
Modern varible vane turbos are quite economical when driven normally, probably as economical (if not more ?) as the normally aspirated versions of the same engine..
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Mercedes Benz - Effect of turbo power - DP
