Why are diesels so poor in traffic, mpg wise?

[rottenegg]

Just out of curiosity really. I have a MK6 Golf 2.0 CR TDI and a MK5 Golf GTI. The diesel Golf can do 55-60mpg easily on a free run, but get it in heavy traffic and the mpg takes a massive nose dive compared to the GTI. Not really sure why since diesels are all about very precise fuel quantity metering, which at idle and traffic crawling you would think would command a minimal fuel demand.

Just interested in the technical reasons as to why if anyone has any insights on petrol vs diesel?

I suppose the back story is I'm interested in buying a 335d for a bit more thrust and fun, but a common observation among owners is terrible urban fuel economy.

[GreenV8S]

Can't speak for those particular cars but in general I'd expect a diesel to do comparatively better under idle / light load conditions because it has much lower pumping losses than a petrol engine.

[stevieturbo]

rottenegg said:

Just out of curiosity really. I have a MK6 Golf 2.0 CR TDI and a MK5 Golf GTI. The diesel Golf can do 55-60mpg easily on a free run, but get it in heavy traffic and the mpg takes a massive nose dive compared to the GTI. Not really sure why since diesels are all about very precise fuel quantity metering, which at idle and traffic crawling you would think would command a minimal fuel demand.

Just interested in the technical reasons as to why if anyone has any insights on petrol vs diesel?

I suppose the back story is I'm interested in buying a 335d for a bit more thrust and fun, but a common observation among owners is terrible urban fuel economy.

they are not.

diesels use far less fuel at low loads than a petrol engine.

And very obviously any engine running and not moving....will result in far less miles per gallon, because you're sitting going nowhere but still using fuel.

[mmm-five]

What is the difference in MPG between 70mph cruise and 30mph stop-start for both your GTD and GTi?

My BMW 320D can get 55mpg on a 70mph long run, or as low as 35mpg driven in stop-start London traffic...so about 64% of the efficiency in stop-start traffic vs cruising (although have not seen below 35mpg even when I'd done nothing but hooning around the NC500 or a track day).

My BMW Z4MC can get 32mpg on a 70mph long run, or as low as 16mpg driven in stop-start London traffic...so about 50% of the efficiency in stop-start traffic vs cruising (and have seen 8mpg when doing a track day at a small/twisty track where 3rd was the highest you'd ever get).

However one is a turbo-diesel, the other is an NA petrol, and I've got no turbo-petrol car to check.

Edited by mmm-five on Saturday 6th August 11:01

[Pica-Pica]

Many taxis were diesel in the past, because of longevity and fuel savings. I did not buy my 335d for town driving, even then it uses little about town. Its long term average is about 44 mpg, and gets over 500 miles a tank, between full and 50-miles-left warning light. If you get lower than 35mpg out of a 335d, you are either ragging the arse off it everywhere, or you really should have a town car.

[Skyrocket21]

The CR shouldn't drop below 40mpg in urban driving, it's all about gearing and the 30% extra energy from diesel, using the turbo and torque peak, which you can't get to in traffic but add twin turbos and big nozzles and more cylinders you're going to get huge power but use more fuel, it has a lot more power in the BMW though, alot on how your right foot manages it.

So diesels come into their own on cruising at speed, especially if aerodynamic too, more torque, higher gearing, and that extra energy, the drop in urban areas isn't much different to a petrol, I guess a hybrid electric or small engine etc would be the winner in traffic or stop start.

[Wills2]

I get mid 40's MPG from my MHT 730d in traffic urban driving, I think that's pretty good.

[Scrump]

I understand that generally diesels take longer to warm up than petrols. Shorter urban journeys would therefore see worse mpg while the engine reaches operating temp.

[GreenV8S]

Scrump said:

I understand that generally diesels take longer to warm up than petrols. Shorter urban journeys would therefore see worse mpg while the engine reaches operating temp.

They take longer to warm up because they are so much more efficient under light load. That's a reason to expect better MPG than petrol under those conditions, not worse.

[Tony1963]

GreenV8S said:

They take longer to warm up because they are so much more efficient under light load. That's a reason to expect better MPG than petrol under those conditions, not worse.

Once warmed up, yes. While still colder than optimum, no.

One way to show yourself this is to leave a diesel car idling from stone cold in winter. It’ll take bloody ages to warm up. A similar petrol engine might hit the 90c coolant temp in ten minutes or so, a diesel might take an hour.

[E-bmw]

Tony1963 said:

GreenV8S said:

They take longer to warm up because they are so much more efficient under light load. That's a reason to expect better MPG than petrol under those conditions, not worse.

Once warmed up, yes. While still colder than optimum, no.

One way to show yourself this is to leave a diesel car idling from stone cold in winter. It’ll take bloody ages to warm up. A similar petrol engine might hit the 90c coolant temp in ten minutes or so, a diesel might take an hour.

Correct, simply because "more thermally efficient" which for short distances is why diseasels are not ideal.

[GreenV8S]

E-bmw said:

Correct, simply because "more thermally efficient" which for short distances is why diseasels are not ideal.

Not ideal only from the point of view of passenger comfort in cold weather. Which many manufacturers address by addiing a separate diesel cabin heater. Heating the cabin directly is much more efficient for short journies than heating the whole engine and using that to warm the cabin.

[Pica-Pica]

GreenV8S said:

E-bmw said:

Correct, simply because "more thermally efficient" which for short distances is why diseasels are not ideal.

Not ideal only from the point of view of passenger comfort in cold weather. Which many manufacturers address by addiing a separate diesel cabin heater. Heating the cabin directly is much more efficient for short journies than heating the whole engine and using that to warm the cabin.

A FFH (fuel-fired heater), requires a different FDM (fuel delivery module). Usually built in from new, but can be retrofitted. A common option on commercial vehicles.

[E-bmw]

GreenV8S said:

E-bmw said:

Correct, simply because "more thermally efficient" which for short distances is why diseasels are not ideal.

Not ideal only from the point of view of passenger comfort in cold weather. Which many manufacturers address by addiing a separate diesel cabin heater. Heating the cabin directly is much more efficient for short journies than heating the whole engine and using that to warm the cabin.

That too. Although I have recently bought a v50 d2 & that seems to warm up way quicker than Mrs E's previous diseasel CoCar.

[stevieturbo]

Either way, the simple fact still remains that diesels are not poor in traffic.