It's never as straight forward as it looks....
Diesel engines have a low specific power output (eg kW / litre of displacement) - partly due to their lower max revs. This is why diesel engines typically tended to be larger than petrol counterparts (eg 1.9 diesel is about the same as a 1.4 petrol).
The great thing about diesels is that as they are compression ingnition anyway you can just go on increasing the inlet pressure (which is what turbo charging does) and the fuel input to extract more power out of a given unit. Of course you eventually reach the limit where the engine self destructs. Doing this with petrol engines is more difficult as you run into problems with the compression ratio causing auto-ignition problems (pinking) which damages the engine.
The other good thing about turbocharging a diesel is that the engine's efficiency goes UP as the boost pressure goes up! ie a turbo'd engined making the same power output as a normally aspirated engine will be using LESS fuel (assuming all else is equal). This gets even better if you cool the air coming out of the turbo (inter- or charge- cooling) and is borne out by the PSA HDi engines where the 110 is typically 2 or 3 mpg better on economy in similar applications than the 90 (where the principal difference is the intercooler) - despite the 20% hike in power (good eh - more power for nothing!).
All modern diesels will be turbo charged in order to minimise engine size for a given power output. Although I think you can still buy N/A versions of VAG's TDi unit and PSA's XUD I don't think they will last much longer.
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RichardW
Is it illogical? It must be Citroen....
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If a diesel engine's efficiency goes up when using the turbo does it follow if you choose a lower gear and higher revs for a given road speed you will use less fuel while you are in boost rather than using a higher gear and not having the turbo kicking in? Or does the difference just apply between a turbo and non turbo version of the engine at the same revs and road speed with and without the turbo in boost?
teabelly
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All of the above AND it improves scavenging - scouring the waste gases out allowing more effective air charge in.
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All of the above AND it improves scavenging - scouring the waste gases out allowing more effective air charge in. >>
Which, in turn, improves emission figures. For this reason, all diesel engines are likely to be turbocharged in the near future as emissions regulations become even tighter.
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But reducing engine size reduces the level of low down torque. I've driven both the 1.9 SDI and 1.4TDI Polo's, and prefer the torque on my 1.9. On the other hand, the 1.4TDI's performance (speed/accelaration) is much better than an NA diesel.
(both have similar PS and peak torque fiqures, but the torque band is narrower IMO)
The other thing I have found is that SDI's are more economical around town, and TDI's better on long runs (for a given engine size).
As for NA versions going, VW has a 2.0SDI, PD? in the new golf, so there must be some life left in them.
Just my 2p's worth
Joe
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OK, if turbos add efficiency and cleanliness, why don't manufactures downsize the diesel engine and increase the size of the turbo (or even put on multiple turbos)
On an aside why are diesels turbocharged rather than supercharged?
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Just increasing the size of the turbo could increase turbo lag to an unacceptable level and put even more strain on the engine. A turbo diesel can be running a static C/R of 18:1 but under boost this will rise significantly which leads to reliability problems such as blown head gaskets, etc. It's only recently we had the joy of turbocharging with low lag brought about by experience, electronic fuel injection and varable geometry turbos. Just look back at the old Saab 99/Porsche turbo reviews to see how off putting turbo lag can be, I know they're not diesel but it illustrates the point.
Steve.
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Having recently driven a new 406 HDi I found the turbo lag in that pretty unacceptable after driving a NA petrol car normally (of similar power). The throttle response was pretty poor and made getting out of a black ice incident far more hairy than I would have liked.
As glowplug says, I think a smaller engine with a bigger turbo would have made the throttle response and lag even worse and made it a really nasty drive. I've driven a couple of NA diesels before (a Transit van and a 106), and while they were both slower than a sloth on valium, at least the throttle response was reasonably good, and a sharp stab on the gas gave a rewarding surge (of pretty small proportions it must be said!).
I've never driven a turbo petrol car I'm sad to say :(
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Having recently driven a new 406 HDi I found the turbo lag in that pretty unacceptable after driving a NA petrol car normally (of similar power). The throttle response was pretty poor
The HDI engines don't have much throttle lag unless the EGR valve is faulty. The HDI is a nice engine even though the design of the 2.0 90 and 110bhp engine is 6 years old.
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On an aside why are diesels turbocharged rather than supercharged? >>
Turbochargers use power from the exhaust gases as they pass through the exhaust system, i.e. power which would otherwise just go to waste. Superchargers are mechanically driven from the crankshaft and the power required to drive them has to be paid for in fuel consumption. Superchargers also tend to generate more noise.
Supercharging used to give much better lowdown power than turbocharging but the recent developments already mentioned by Glowplug have vastly reduced the difference.
There are probably some advantages in supercharging two-stroke diesels but we don't have any of these in the automotive field.
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