What no one has said so far is the benefit of the turbo.

On 1.0’s or 2.0’s they make petrols more driveable ie give more low down torque, so for more day to day driving they just drive nicer than old petrols, which tended to need revving hard to get performance.

The Ford eco engine sounds pretty good when you rev it hard. Then again I’ve always thought 3 cyclinder engines do!

As to mpg, I can never get more than 44mpg out of our 1.0 100bhp Fiesta on a commute. However this compared to about high 30’s in aToyota Aygo 1.0, high 40’s in a 2.0 diesel or now low 50’s in a 2.0 petrol hybrid.

My commute is 20 miles each way and hilly, so it kind of proves that small engines are not always most efficient

Checked every 2 months, they are sitting between the normal and economy pressure 36 instead of the 39 for most economic.

Got a 1.4turbo Astra (150bhp) on lease. Just about to hit 3000 miles. Average mpg since new is 40mpg with careful driving. If you press the accelerator down alot instant mpg hits single figures easily.
My old 2.4 200nhp desiel alfa would do 32mpg being thrashed all day.
Wasn’t a service item on the 150bhp eco boost the engine block (100k miles iirc)?

It is ‘old’ as in 30k run in…I’d expect a slight improvement on mpg over the length of time I’ve owned the car as I think the driving behaviour should adapt to the car and engine…suspect I haven’t adjusted as I expected and it is a slightly heavier foot than before.

It’d still be in the garage if it were me. You’re talking about typical instant mpg whilst using crusie on the same road…

I’ve been looking at the fiesta, but round my way they are highly sought after by thieves for the st and the eco model, that hikes the insurance up, and as I have a ds3 at the moment was curiously looking at the new new c3 which looks pretty great, and have already found 4 nearly used ones just over £11k which is a bit of a bargain running the 1.2ltr 110bhp engine, just seems a step down from my 1.6 ds3 but saying that can’t see any nearly new cars without these eco engines.

I like the engine, the turbo is good…however it needs a wee bit of revs to go comfortably.

I’ve got access to a couple of cars, all non-European and they all prefer changing gear sooner than the turbo…the efficient driving thing indicates a gear change and it is fine. On the turbo, 20mph in 3Rd isn’t smooth in the turbo…when the turbo is kicking it is fantastic.

Well the VW one in my Dad’s car is 4cyl rather than 3, which would make it smoother.

He gets about 53 or so on a long run, averaging about 48 with a mix of short town trips and longer runs.

Got a 1.4turbo Astra (150bhp) on lease. Just about to hit 3000 miles. Average mpg since new is 40mpg with careful driving.[/I]

As a comparison my previous car, a Vectra 1.9 diesel AUTO averaged 43.5mpg (according to the trip computer zero’d on the day I bought it) over the 50k and 3 years I had it, with no attempt to drive economically.

I don’t know about the Ford engine, but my OH has a 1.5 3cyl Mini Cooper, which does 42-45 mpg on her commute (computer says 50+). Mixture of roads, she drives as fast as the roads allow, I guess. It goes well and sounds good, too.

£20 part, but you need an expenisve set of presses and a mechanic to replace it.

Two trusted local garages and main stealer said they did not have the press, so always buy hub.

Having spent daft money on keeping the Touran and Passat going, I lean back towards the simpler Seat 1.4 16v and our 53 Yaris D4D for low cost parts and less to go wrong in the first place.

I take your point about cars in the past being knackered by lower miles over lifetime.

However, you are a good home mechanic, I’m not. New cars cost more at garages to fix.

That bit didn’t quite ring true (and it’s not a VAG quote, it’s from Reuters). The direct injection engines work on the principle that you can have part of the combustion chamber with a stochiometric mix and part just air, so as the mix burns it pushes down on the air, which pushes on the piston. Which is more efficient than running the engine with less air like a conventional engine. It also avoids NOx because the excess air (where NOx comes from) isn’t heated. When you floor it, it reverts back to normal and just fills the combustion chamber with a stochiometric mix (i.e. it’s richer, but not necessarily over fueled?

That’s not quite right – a petrol engine will always try and run at Lambda 1, ie the correct ratio of fuel to air so that all the fuel burns. If there were excess air it would be above Lambda 1 and temperatures and NOx would skyrocket. When the engine gets hot, especially with turbos, the Lambda is reduced (more fuel) with the excess fuel used to cool the exhaust system down. Typical Lambdas for this can go down to around 0.8, but high power vehicles will see it go down to 0.7ish.

That’s not quite right – a petrol engine will always try and run at Lambda 1, ie the correct ratio of fuel to air so that all the fuel burns

No, it is right. It’s a stratified charge engine. So under partial load the cylinder isn’t a homogenous fuel/air mix, there’s only fuel and air at one end.

https://en.wikipedia.org/wiki/Gasoline_direct_injection

New cars cost more at garages to fix.

They do often, but not always. But we are (usually) getting more for our money, it’s not some kind of conspiracy.

It’s still running Lambda 1 though, there is no excess air in the cylinder.

Lean burn, where there is excess air, is very very different to normal direct injection and requires extra exhaust after treatment to cope with the NOx from the significantly higher cylinder temperatures and excess air.

(Hate to do this, but my day job is calibrating petrol engines for major automotive OEMs so I can assure you this is correct)

Timely thread. Anyone have experiences of these things in larger vehicles?

My lease is up next month (actually, it was up on the Hyundai last year, I’ve got a stay of execution by way of a Passat CC till March). I’m looking at a Mondeo, and as I do far fewer motorway miles these days I was looking at the 1.5 Ecoboost rather than the traditional 2.0 diesel lump. Am I going to regret this?

I’ve got a 2 litre Ecoboost. It is splendid. 247 bhp and 254 ft lb of torque from 2 litres and averages 34 mpg with mixed driving. Pretty remarkable.

Back on-topic; I know people with the 1.0 litre Ecoboost, in Fiestas and they love them. Lots of MPG if you drive sensibly and plenty of grunt for such a small engine.

That Wikipedia page isnt quite describing direct injection correctly – it is basically saying DI engines are lean burn which isn’t true. Essentially

Lean Burn EQUALS Stratified DI

Stratified DI DOES NOT ALWAYS EQUAL lean burn

It’s still running Lambda 1 though, there is no excess air in the cylinder.

It’s not the same lambda throughout the cylinder. So one end lambda is 1, at the other it’s er.. inifinty.

Given your credentials, can you explain how it actually works then if I am wrong?

Stratified DI DOES NOT ALWAYS EQUAL lean burn

I think it says that, second half of first section – three modes of operation?

I’m seriously thinking about abandoning my “buy a car for < 4k” attitude and going new/nearly new. All this efficiency stuff doesn’t seem great as cars age.

All my recent cars have had issues caused by EGR valves for instance. Not just the EGR valve itself, but clogged air intakes, swirl flaps and valves on direct injection engines clogging with soot and muck.

Current 2.0 TFSI has what sounds like a slight misfire as the valves are sticking and the only way to cure it is to take the top of the engine to bits and clean the valves by hand. 400 quid at an independent. 🙁

It’s a tax on the poor!

It’s not the same lambda throughout the cylinder. So one end lambda is 1, at the other it’s er.. inifinty.

Given your credentials, can you explain how it actually works then if I am wrong?

You are right that it layers up the Lambda throughout the cylinder, but the reason for this is to give a rich pocket next to the spark plug. The total in-cylinder Lambda is still 1 for normal running in pretty much all petrol engines and so there is no excess air. DI allows you to time a rich pocket next to the spark plug for whenever is best for complete combustion of the entire cylinder charge. It can also then be used for other functions such as running a lean mixture throughout most of the cylinder and then a late main inejction to make a really rich pocket near the plug – this is used for knock control as the lean mixture cannot auto-ignite whereas the rich pocket is ignited by the spark plug. However, the in-cylinder Lambda is still <=1.

Lean burn is where Lambda > 1 and a few manufacturers have engines with it (Mercedes being the main one but I don’t think it’s in production any more). This runs with excess air in the cylinder allowing you to run with the throttle open and reduce pumping losses. This is great for efficiency but is horrific for NOx and as such you need to run with an LNT or SCR in the exahust to mop up the extra NOx created.

Component protection is then where Lambda < 1 for the purpose of cooling the exhaust using excess fuel. Lambda can drop to around 0.7 depending on the temperature drop required. In extreme examples, the DI can be used to inject fuel in the exhaust stroke to cool the exhaust. This allows the combiustion injection to be leaner (around Lambda 0.85) to give more torque, but there is still fuel delivered to cool the exhaust.

Hope that explains a bit – happy to answer anything else if required!

So it only saves fuel by allowing increased compression ratio and hence improving thermal efficiency?

Why don’t they solve the NOx problem by filling the cylinder with EGR then injecting both air and fuel into a pocket?

I thought the idea was the fuel was injected very late (after TDC), so that the lean parts of the cylinder weren’t hot enough to generate NOx?

(My background knowledge is in burner design, which is obviously a bit different to how a cylinder behaves, but still relies on adding fuel to the correct parts of the flame to minimise NOx)

So it only saves fuel by allowing increased compression ratio and hence improving thermal efficiency?

Essentially, yes. DI by itself in a normal, homegenous engine (i.e. 99% of those in the market) doesn’t really do anything itself – what it does is enable other technologies which can help efficiency. One is higher compression ratio which DI enables by allowing greater scope in injection/ignition timing and also charge cooling through the evaporation of the fuel in the cylinder itself. Another is allowing use of larger amounts of valve overlap to capture EGR and allow a more open throttle (see below…). It also helps with more accurate metering, especially in transient situations where PFI metering gets fairly inaccurate

Why don’t they solve the NOx problem by filling the cylinder with EGR then injecting both air and fuel into a pocket?

EGR is used for this, but for an efficiency benefit rather than NOx reduction. If the engine needs to operate at part load, then to run at Lambda 1 you need to shut the throttle partly to reduce the air load in to the cylinder – this causes pumping losses which reduce efficiency. By using EGR (mainly through valve overlap changes, but can also be an EGR valve like a diesel as well) you can ‘pre-fill’ the cylinder with EGR so that with a fully open throttle you still end up with a reduced air charge in the cylinder – open throttle = lower pumping losses = better fuel efficency.

In a lean burn engine, the whole point is that you run with the cylinder with excess air to allow leaner conditions with the fuelling required for combustion so this reduces your scope for EGR use, however it is still used. The in-cylinder temps in a gasoline engine are significantly higher than a diesel though so it’s much harder to use EGR to lower combustion temperature to reduce NOx, hence the need for aftertreatment.

I thought the idea was the fuel was injected very late (after TDC), so that the lean parts of the cylinder weren’t hot enough to generate NOx?

In a gasoline engine you wouldn’t be able to put the main injection that late and make any torque – the air and fuel needs to mix before it can be ignited around TDC (obviously depending on spark timing). Even with a stratified charge system there is still a good amount of charge mixing, you just keep injecting fuel later and later to put a richer pocket near the top of the cylinder.

In a normal homogenous charge engine, because all excess air is used up in combustion (or should be!) the NOx contribution is tiny over a cycle so there is no real need for directly calibrating for it (as such, clearly we make sure it is as low as possible, but CO and HC emissions are much more significant).