[Closed] Diesel fuel economy on shortish heavy traffic trips?

A diesel will take longer to warm up on these trips which will affect the fuel consumption negatively. How long are the journeys and what size car?

I get 55-61mpg on motorways, about 38-45 on bad urban runs. That's without start/stop technology, but I seem to spend little time actually stationary so I dunno how much that helps.

Top top for town driving - get a hybrid. Mine goes from 55-60mpg on motorways down to about 52-56mpg. Much less drop.

Some are good, and some not so. My Mum has a 208 and all her trips are short, manages +55mpg. How short are we talking?

You need to really work out all the numbers to know whether swapping cars will work out financially worthwhile, not just mpg.

I doubt it's going to return less mpg than the petrol - but it's going to need a blast/long run every now and then.

If you get one with start-stop technology, that can make a big difference.

PS my 38-45mpg is from my auto passat, so a big car.

Aren't short trips in heavy traffic what TaxiCabs do?

When's the last time you saw a petrol TaxiCab?

Short traffic dense journeys are supposed to be a diesels forte .

My diesel BMW 120D was pretty shocking (for a diesel - 35mpg) in short, traffic-ridden journeys.

My 'new' car (a 1.4 petrol Megane) does 45mpg on the same journey.

Short traffic dense journeys are supposed to be a diesels forte .

No they aren't. They're the worst thing for diesels.

Diesel forte is long motorway trips.

Yeah, short trips definitely not a diesel's best mode of operation.

If it is primarily going to be used for short journeys, then a small, boosted petrol engine will get you far better economy on the short journeys and won't be far off on the longer ones.

Small boosted petrol is what I've got (golf tsi).

The economy isn't that far off a diesel on longer A road type journeys, but I wondered if diesel would be expected to drop a similar 25% on a 40min heavy traffic trip?

As Sharkbait said until the engine warms up a diesel is pretty dire so expect to lose 1/3 in mpg.
Diesel taxis are running most of the day so the engine is always warm.

A small car with midsize or torquey engine is what would suit the journey best or a moped.

Short traffic dense journeys are supposed to be a diesels forte .

No they aren't. They're the worst thing for diesels.

Diesel forte is long motorway trips.

An efficient engine is an efficient engine and to suggest it is the worst thing for diesels [ cold start is bad for all engines so that aside] is meaningnless - though it is stw dso I expect someone to try.

In essence a car that does good mpg on motorways also does it urban as it is an efficient engine

Given fuel costs and additional insurance costs of a diesel a very small engined petrol engine is probably the best option [ ignoring a hybrid] balancing costs and mpg

Small boosted petrol is what I've got (golf tsi).

The economy isn't that far off a diesel on longer A road type journeys, but I wondered if diesel would be expected to drop a similar 25% on a 40min heavy traffic trip?

Think the same engine but smaller car. Most of MPG is lost on setting off and pully the extra bulk from standstill.

Aren't short trips in heavy traffic what TaxiCabs do?

When's the last time you saw a petrol TaxiCab?

When did you last see a taxicab with it's engine off?

There's a more significant disadvantage to diesel on short trips - the engine is more likely to get coked up and break expensively.

In essence a car that does good mpg on motorways also does it urban as it is an efficient engine

True but the ratio of urban/open road economy varies.

Hybrid ftw.

An efficient engine is an efficient engine and to suggest it is the worst thing for diesels [ cold start is bad for all engines so that aside] is meaningnless - though it is stw dso I expect someone to try.

I see your point, but that isn't strictly true. An engine only reaches it's peak efficiency at a reasonably narrow temperature, speed, and load range. Outside of that it is down to the engine design and calibration as to how good it is (cold start, warm-up, high load, low engine speed, etc). A gasoline engine will get to it's most efficient point in terms of temperature and also the speed/load range being used more quickly than a diesel for short, urban driving so it is more efficient. As the journey starts to get longer and driving turns to crusing rather than stop-start, that is where a diesel will be operating in it's most efficient band and you see diesels move away in terms of economy.

Hybrid ftw.

Bike ftw?

Small boosted petrol is what I've got (golf tsi).

The economy isn't that far off a diesel on longer A road type journeys, but I wondered if diesel would be expected to drop a similar 25% on a 40min heavy traffic trip?

Sorry, missed that bit! If that is the sort of driving, then it would be a bit of a toss-up between the two. I doubt you'd see a 25% drop in fuel consumption. The diesel would be less efficient to start with, but when warmed up the torque curve could be to your favour in traffic. On the other hand the petrol will be more efficient to start with but endless pulling away and stopping would drop the efficiency a bit.

gasoline engine will get to it's most efficient point in terms of temperature and also the speed/load range being used more quickly than a diesel for short, urban driving so it is more efficient.

The reason why petrols warm up more quickly is because they are LESS efficient, and more heat is wasted. However this is counteracted by the fact the engine warms up more quickly as described 🙂

PS glenh a small engined diesel golf will be good for 60mpg easily enough so your 45 is a fair bit lower 🙂

The reason most petrol engines warm up quicker is because they don't have cast iron cylinder heads to cope with the increased compression.

My 1.9tdi (Audi a4) does low 20's until its warm then it'll do 55mpg easily all day long. I only do A roads so it warms in a few miles, starting cold around town it probably wouldn't warm up on a short journey. As said above, will at least cause issues with DPF if not other more expensive issues. Get an aygo/c1/107 thing for town stuff

I realise its annoying see 35mpg on the display, but you've got to balance this against the fact you're doing less miles so it isn't necessarily costing a fortune? If it was measured in gallons per hour (probably more relevant in traffic?) or something you'd probably be doing quite well.

I wouldn't buy a diesel for the journey you describe either. Small car, with a small petrol engine. Or a hybrid.

The reason most petrol engines warm up quicker is because they don't have cast iron cylinder heads to cope with the increased compression.

Don't think so - it's less wasted heat.

Think about how combustion occurs in a diesel. You've got a charge of air, and fuel is injected at the top of it. If you're driving slowly you aren't injecting much fuel, so you've got just a tiny little spurt of diesel burning in a relatively large bit of air, so it burns and expands, and the combustion products are pretty cool by the time they get to the cylinder walls and even cooler by the time they are on their way out of the cylinder.

In a typical petrol engine (although not in your TFSI engine actually) the whole cylinder is full of air/fuel mix, all of which burns, including the bits in contact with the walls, so you get a whole charge of hot combustion products heating the whole engine up. So more heat gets transferred to the engine components which then gets removed via the coolant.

An idling petrol engine uses six times more fuel than an idling diesel, I remember reading somewhere. So a sixth of that energy is going to turning the engine over, and the rest is energy going straight into the coolant.

molgrips speaks sense.

did we get an answer to " How long are the journeys?"

As I said earlier , on long runs the difference in mpg between petrol and diesel isn't that great but on shorter journeys a diesel is significantly better .

As I said earlier , on long runs the difference in mpg between petrol and diesel isn't that great but on shorter journeys a diesel is significantly better .

Sorry, that is still the wrong way round. On long, cruising journeys you will, generally speaking, get better fuel economy out of a diesel than a petrol by quite a bit. On medium, less cruise-y journeys there probably isn't a huge amount in it with modern engines. As the journey gets shorter, the petrol will get better fuel economy than the diesel.

Molgrips is pretty much bob on with his explanation. A he mentions, the weird thing is that a petrol is less efficient whilst warming up, but because it gets there quite a bit quicker than a diesel then it evens out to actually be better whilst the journey is short!

Aren't short trips in heavy traffic what TaxiCabs do?

When's the last time you saw a petrol TaxiCab?

when was the last time your passengers were paying you £5/mile. Fuel economy counts for less than longevity and reliability for taxi owners. Diesels have better low rev torq so crawl in slow traffic better than petrol engines. When I used to do multi drop work in london we had one diesel and one petrol van - riding the clutch on the petrol on in creeping traffic for hours was agony (and non too healthy for the van) where as you could drive in gear at slower than walking pace in the diesel

Think about how combustion occurs in a diesel. You've got a charge of air, and fuel is injected at the top of it. If you're driving slowly you aren't injecting much fuel, so you've got just a tiny little spurt of diesel burning in a relatively large bit of air, so it burns and expands, and the combustion products are pretty cool by the time they get to the cylinder walls and even cooler by the time they are on their way out of the cylinder.

Are sure about this because last time I touched my exhaust by accident after the car was running it took the skin off and I'm sure that was because it was hot from the exhaust gases passes through it which hadn't cooled down by then. Don't forget that oil has to warm up too, even multigrade, and most diesels require more oil so more energy to warm. Add to that the thermal expansion of the parts inside the engine as they reach their ideal operating temperature and the tolerances they were designed to run at. The coolant then tries to keep the engine at this ideal operating temperature. Back to what I stated diesels require more time to warm up because of heavier components in the engine and also more oil.

The following probably explains it much better

Diesel engines are different than gasoline powered engines and have both operating and starting techniques that also are different. Gasoline engines burn gasoline, which is fairly volatile and flashes easily. All it takes is a spark from a spark plug at the right time and the cylinder fires. Diesel fuel is much heavier than gasoline and doesn't burn as readily. But, when it does burn, it produces more BTU per gallon than gasoline, which results in greater power and efficiency. A diesel engine fires by compressing the air-fuel mixture to a very high compression ratio, then it fires by itself, without any ignition system or spark required. All it takes is for the fuel to be injected into the cylinder at just the right time and amount. If a diesel engine is real cold, that cold will affect it in two ways. First of all, the drag on a cold, stiff engine will cause it to crank over much slower than a warm engine. All that mass and heavy oil really tends to slow it down and inhibits it's firing process. Secondly, the diesel fuel (just like gasoline) flashes off and fires once it reaches it's ignition temperature. On a gasoline powered engine a spark plug does this but a diesel is relying on the heat generated by compressing this mixture very tightly. If the air-fuel is cold or the cylinder is cold, it may not fire.

Losses due to friction, and the thermal efficiency of an engine are two entirely seperate issues.

Your right in that a diesel will have heavier components to deal with the increased combustion pressure and torque it will produce (nothing to do with compression ratio) and clearly when cranking and cold it will take a bit more energy to get the thing spinning, but that is what a second or so?

A diesel engine is more thermally efficient than a gasoline engine meaning that more of the energy from combustion goes into actually pushing the piston down rather than warming up the engine. This is why a diesel takes longer to warm up than a gasoline - as Molgrips mentioned, it's because a Diesel is TOO efficient which means it doesn't get to it's most efficient operating point as quickly (a weird paradox I know). Your right in that the exhaust is clearly very hot, but that is because even the best diesel engine is only about 35% efficient - that is still a LOT of heat coming out of the exhaust!

EDIT: Or to put it another way, if you had a diesel and a gasoline engine of the same rotating mass, same block/head mass, same oil volume, same coolant volume etc (basically two identical engines), the diesel would take longer to warm up than the gasoline would.

Are sure about this because last time I touched my exhaust by accident after the car was running it took the skin off

It will get hot, just much more slowly. It takes bloody ages, 20 minutes maybe, for a diesel engine to get warm just by idling. If it's cold it may never get warm. In cold countries if you leave a diesel taxi idling for a while the temp gauge drops and the engine actually gets colder than operating temperature.

craigxxl the stuff in your quote is correct, but doesn't explain the longer time taken to warm up.

As for there being more engine oil - there is only a little more, and oil has a far lower specific heat than the water in the coolant. Oil gets up to temperature much more quickly than the coolant in engines generally.

i cannot understand how the mpg between the two is so widly different on long journeys but closer artound town? On a normal legal speed driven run my 1.8 kompressor gives me around 48-50 mpg , steady foot and no hard acceleration but around town it sinks to 27-30, which i think is about right. My other car, a 1.9tdi gives me about 60 on a run but easily 40 around town even driven with not much concern for the consumption so it leads me to think that a diesel is better around town and a petrol catches back up on a longer steady run.

I Have an astra H estate 1.9cdti,sri 150bhp but remaped to 190 on the sport button and eco off the sport button. I get 58 to 68 mpg on the motoway sometimes more. but around town it is terrible even in eco i get between 38 to 45.and only if i keep the egr valve clean,this costs about £6 every 6000 miles and takes about 30 mins once you know what your doing.

My old 1l punto was way better round town.

But you are comparing nearly a two litre car with a one litre . My Gf 1.2 corsa is way better around town than my merc but not as good on a long run as its revving too high at 70-75.

ISTR something about modern small petrol engines having their cats closer to the engine so that they warm up quicker from cold. Problem being that they then run way too hot on a run, so the engine has to run rich to cool the exhaust gases - and it does this by squirting in more petrol so further reducing economy.

But you are comparing nearly a two litre car with a one litre . My Gf 1.2 corsa is way better around town than my merc but not as good on a long run as its revving too high at 70-75.

And to be honest you have hit the nail on the head with this as it's frankly impossible to just say from the fuel which is better from a situation - but the generalisations given above are still correct. There are some engines that will break those generalisations. We've got a couple of gasoline engines at work that blow diesels out of the water for fuel economy, and some diesels that are very efficient when cold. But they have some pretty high tech stuff in.

ISTR something about modern small petrol engines having their cats closer to the engine so that they warm up quicker from cold. Problem being that they then run way too hot on a run, so the engine has to run rich to cool the exhaust gases - and it does this by squirting in more petrol so further reducing economy.

Yes, most petrol engines will now have a close-coupled (or close-coupled-ish) catalyst, so that it heats up quickly to get the emissions down as quick as possible. You also have a component-protection fuelling for when the engine is running fully-loaded (or wherever it is determined to be necessary) and things like turbos, cats etc are getting a bit toasty. You wouldn't see this on a normal drive though (well, depends how you drive I suppose.....)

i cannot understand how the mpg between the two is so widly different on long journeys but closer artound town?

It's short trips vs long trips. If your car takes 10 mins to warm up, that's 66% of a 15 minute drive spent under temperature, but if it takes 3 mins to warm up it'd only be 20% of the drive spent cold.

but the generalisations given above are still correct

Yes. In basic terms, diesel:

- is more thermally efficient as a heat engine due to greater compression
- has more energy per litre
- can burn a tiny bit of fuel, whereas petrol has to have a certain amount of fuel in the cylinder for the spark to ingite it

But there are many more factors involed in some specific situations.

Speed12 - interested in what engines you are talking about. Car ones or some specific apps?

They are prototype engines so can't really say much about hem, but they are for car applications. Probably a generation or so away from production, but based on current engine models with fancy tweakery to fuelling and boost systems, and some interesting warm-up strategies and bits and bobs to make them more efficient when not up to temp.

It's all going very downsized petrol engine on the whole though - small, highly boosted petrol engines can get you all e benefits of a small petrol, plus most of th economy benefits of a diesel. Look at engines like the new Ford Fox 1.0l - amazing little engine that is driving like a 1.8-2.0l but giving you the fuel economy of a 1.0l. Same with Fiat twin air. Most of the big manufactures have a similar one coming up. Even BMW will be using a 1.5l turbo petrol as their base engine in the very near future.

How realistic are the small petrol engines though? I mean, I always thought it was a great idea and solved a lot of issues but the VAG ones and the Fiat Twin Air seem to be rather difficult to get good economy from.

Incidentally, Toyota went from 1.5l to 1.8l in the MkIII Prius to [i]improve[/i] fuel economy. They also send the coolant through a matrix in the exhaust to warm it up more quickly.

I hope you are implementing some of the ideas I've had for fuel economy 🙂

The anecdotal evidence is that my fiance's 1.4 TDI Polo is much more economical that her 1.2 16V clio on a 14mile stop-start commute. The journey is quite hilly so I think the extra torque is a big factor. (I wish the Polo had a 6 speed box for extra motorway legginess.)

I'm not sure this type of commute is good for any sort of engine really.

1.4 diesel Polo bluemotion here - 50 mile round commute everyday. 65 MPG on A roads, 55 MPG on rural roads and 35 - 40 MPG in the short 2 mile stretch of stop, start traffic. Generally get between 55 - 60 MPG average over the journey depending on traffic.

You obviously want to save money, which you won't do by buying a different new car.
You need a bag o' shite old one, that is light, reliable and easy to drive in traffic.
You need a MkII Nissan Micra, 1.3, wait for it, CVT auto.

Sorry.

Diesels are generally pants for short journeys or heavy traffic. Can't believe that anyone would argue to the contrary, no matter what the efficiency says. Continual use at low loads on a Diesel engine is a good way to incur very hefty service bills that are simply not an issue with petrol cars.

The anecdotal evidence is that my fiance's 1.4 TDI Polo is much more economical that her 1.2 16V clio on a 14mile stop-start commute.

15 miles is quite a long time in heavy traffic, so the warmup factor is less of an issue.

I wouldn't say that diesels are particularly bad in heavy traffic as long as they are warm.

ONLY heavy slow traffic and no higher speed driving is a bad idea though, due to coking issues.

Incidentally, this morning's commute was 11 miles, involving a couple of traffic lights, some 70/50mph dual carriageway and the M3. It took about 8 miles and 12 minutes before the coolant got to the normal 90 degrees ie most of the journey.