Why MPG better on TDI then IDI?

[Jet-A]

Been working on project car (MK1 Jetta 2 door) with MPG as prime objective. I know that as a "general" rule turbo'ing an IDI often increases fuel consumption. I also know that the TDI gets better MPG in cars weighing up to 1000 lbs more. Any theories as to why? Also would it logically follow that a TDI-M engine in a lightweight MK1 yield better mileage? Or are there other variables that I am not considering? I presume that the MK4 Jetta has a lower drag coeffient but I can't imagine that its that significant. I'm also assuming that the gearing may be different but not that much but I am assuming here....thx

[andy2]

The newer cars (MK4) have bigger Tires and the transmission geared to really bring the engine rpm down.Also having the small vnt turbocharger that works well to produce boost at low rpm.The older IDI's with wastegated turbos that had to be reved higher to make the same boost.Having the TDI electronically controlled is where it picks up MPG's over the IDI's too.

It makes me wonder how the 1.9 IDI would work in a mk4 with an electronic pump and vnt on it?I'd bet the IDI wouldn't be very far off at all in terms of "fuel economy".

[OM617]

Direct injection engines are generally 15% more efficient than comparable IDI engines.

[malone]

Direct injection engines are generally 15% more efficient than comparable IDI engines.

Yeah that % figure was quoted from VW.

TDI's increased fuel efficiency is not solely attributed to the MKIV chassis and gearing.. if you drop a TDI in a MK2 with the same transmission it'll still have better fuel economy than a IDI.

There was a discussion in this forum about heat losses in the prechambers of a IDI, which may affect fuel economy. Here's the article.

Also, IIRC when fspGTD (a retired member) installed a VNT-15 from a TDI on his 1.6 IDI he noticed improvements in both power and fuel consumption... compared to the stock T3 turbo anyway.

[mk2diesel]

TDI 's have better control over the fuel delivery ( the pump is electronically controlled)   and 2 stage injectors for a better spray pattern

twix the 2 items , it has better fuel economy and performance .. (and is programable)

[jtanguay]

TDI 's have better control over the fuel delivery ( the pump is electronically controlled)   and 2 stage injectors for a better spray pattern

twix the 2 items , it has better fuel economy and performance .. (and is programable)

i would rather say that it has better performance.  a tdi running in perfect condition should net as much mpg as the IDI in perfect condition.  (in normal driving conditions...)  but the TDI will have more power (acceleration etc.)

[Mark(The Miser)UK]

I suspect that DI diesels have better average mileage, but less maximum mileage.The computer controls 'iron out' differences in driving style. Not met a TDI driver yet over here in the UK who matched my 62mpg in my   1st TD Quantum. I believe there was better to come from my Q as injectors were original AFAIK [160000miles]

[lyeinyoureye]

This is why...



Shame VW doesn't make efficient cars too.