why the move to small engines ?

why the move to small engines ?

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Discussion

StuntmanMike

Original Poster:

11,671 posts

150 months

Wednesday 19th March 2014
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As title really, why is a 4 pot turbo more eco friendly than a 6 pot NA engine, for the sake of discussion lets say a 2 litre turbo 4 cylinder v a 6 pot 3 litre, I dont care what people prefer, or which will last the longest so please dont post with that rubbish, I am asking why one is dissapearing and the other thriving.

greygoose

8,224 posts

194 months

Wednesday 19th March 2014
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Emissions and fuel consumption.

MarJay

2,173 posts

174 months

Wednesday 19th March 2014
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The six pot has wasted energy in the form of exhaust gas pressure. The Turbo takes this wasted energy and returns it back into the system. It's all down to tuning, you can tune a turbo for power or for efficiency. These new engines are just tuned for efficiency. It's a level of efficiency you just can't get out of an NA engine, sadly.

StuntmanMike

Original Poster:

11,671 posts

150 months

Wednesday 19th March 2014
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I see, I always held the simplistic view that a turbo turned a 2 litre into a 3 but that is a very good answer.

zeppelin101

724 posts

191 months

Wednesday 19th March 2014
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In short, it's all driven by fuel economy and emissions.

The larger the petrol engine, the more inefficiently it runs at low load and low speed which is typically where most engines operate on a day to day basis chugging around.

Plus, with engines like V6s, friction is typically quite high whereas friction can be reduced quite healthily in a 4 cylinder which essentially unlocks a little performance and is a nice fuel economy boost at lower speed and load.

The move is to "dethrottle" engines, as pumping losses (the work that is lost getting the air into the cylinder) can be reduced which is free fuel economy. Best way to do that is to use a smaller engine. There are other methods as well, which is why BMW / Nissan / FIAT and a couple of others have a means of using variable valve lift to control engine load rather than primarily relying on the throttle. What that means is that the pressure in the manifold can be close to atmospheric (it can't be exactly atmospheric due to things like purge) which means that the throttle is providing minimal restriction and all of the load control is done through changing the actual valve lift and the duration it is open for.

Alongside that, the overall performance window can be increased using a boosting system to achieve the same or better performance as the larger engine it replaces. Look at the torque curves for a modern 4 cylinder turbo - peak torque is often available from 1500rpm and extends to somewhere between 4000rpm and 4500rpm with peak power not much later. There are very few N/A engines of the class above that can offer such a performance envelope.

In short, take a big engine, get rid of some cylinders and add / improve the boosting system and you end up with a product that can achieve greater fuel economy with the same or better performance and produce less gaseous and noxious emissions.

Before someone says "in my experience, these little turbo engines are crap on fuel" they do require a recalibration in driving style to get the most out of them from a fuel economy point of view. Some are just bad, granted, but moderately sized N/A engines in my experience are less susceptible to driving style in terms of overall fuel economy whereas boosted engines are much more variable due to the available performance envelope and their propensity to overfuel. Conveniently, that is one of the reasons why manufacturers use integrated exhaust manifolds which are part of the coolant circuit so that the window of stoichiometric operation can be increased to higher speeds at full load.

Edited by zeppelin101 on Wednesday 19th March 09:44

StuntmanMike

Original Poster:

11,671 posts

150 months

Wednesday 19th March 2014
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Thanks for taking the time to write all that, excellent post.

kambites

67,461 posts

220 months

Wednesday 19th March 2014
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StuntmanMike said:
As title really, why is a 4 pot turbo more eco friendly than a 6 pot NA engine, for the sake of discussion lets say a 2 litre turbo 4 cylinder v a 6 pot 3 litre, I dont care what people prefer, or which will last the longest so please dont post with that rubbish, I am asking why one is dissapearing and the other thriving.
I can't say I fully understand the physics involved, but a few relevant points:

1) In a primary heat engine (such as a naturally aspirated internal combustion engine) theoretical peak efficiency is determined by the ratio of absolute input air temperature to output air temperature (as demonstrated by the Carnot Cycle). Burning petrol in a typical ICE, this peak efficiency work out as ~37%. A turbocharged engine is not a primary heat engine - the use of exhaust gas inertia can bring engines above 37% efficiency.

2) Whilst the theoretical maximum output of a simple combustion engine is 37%, this decreases dramatically as you throttle the engine back. A typical naturally aspirated car engine running at 50% throttle will be ~25% efficient; at 25% throttle will be <20% efficient. Torque output of a naturally aspirated ICE at a given RPM is generally roughly proportional to engine capacity; thus as capacity rises, the throttle opening required to provide a certain drive force (for example to maintain a constant 70mph) will fall and efficiency will fall with it, this is why smaller engines tend to be more efficient. *

3) A smaller engine with less cylinders will typically have lower frictional losses and will be lighter.



The other huge "advantage" that turbocharged engines have, is a huge amount of flexibility to map how the engine behaves under different circumstances. This makes it much easier to tailor the engine to perform well in specific situations (such as the NDEC cycle) which is why turbocharged cars often perform much better on the official consumption tests than in typical real-world driving situations (although manufacturers are slowly getting better at doing this with naturally aspirated engines, too).


  • Note that this is only true of the fuel mixture remains the same throughout the rev range. In a typical modern engine, the fuel mixture will transition from closed to open circuit at somewhere around 50% throttle, richening the mixture dramatically, which causes fuel economy to start to fall. Thus the actual optimal way to drive for best fuel economy is quite hard to work out, and varies from engine to engine.
Edited by kambites on Wednesday 19th March 10:20

Luther Blisset

391 posts

131 months

Wednesday 19th March 2014
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Great post Zep. Never thought of exhaust manifold cooling being used like that. Clever.
Would it be bypassed at low EGTs?

Do manufacturers still tune their cars super rich? I would have thought they'd have moved on to tech that could get them running between max efficiency and max economy by now, i.e high grade alloys. Even water injection as standard.

Even at large throttle angles they don't need to tune that rich (often 10-11 AFR) do they, they can still make pretty healthy power.

Dumping in fuel for cooling has always seemed a bit of a cheat to me.

zeppelin101

724 posts

191 months

Wednesday 19th March 2014
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Luther Blisset said:
Great post Zep. Never thought of exhaust manifold cooling being used like that. Clever.
Would it be bypassed at low EGTs?

Do manufacturers still tune their cars super rich? I would have thought they'd have moved on to tech that could get them running between max efficiency and max economy by now, i.e high grade alloys. Even water injection as standard.

Even at large throttle angles they don't need to tune that rich (often 10-11 AFR) do they, they can still make pretty healthy power.

Dumping in fuel for cooling has always seemed a bit of a cheat to me.
Overfuelling is a function of the manifold and catalyst temperature generally speaking.

For instance, on an N/A engine there is very little overfuelling other than "performance" overfuelling as there is a slight increase in power running a shade rich at peak power in N/A. Somewhere around 13:1 if I remember correctly.

In a turbo, there is much more heat generated due to the losses associated with using exhaust gas to spin the turbine wheel. This means two things:

  • The manifold runs at a higher temperature due to increased exhaust flow. Ideal gas law is pV/T = a constant. V stays the same, p and T must increase.
  • Downstream the catalyst is also running at a higher temperature.
The restrictions come due to the fact that running the catalyst hot melts the internals and it is then useless at doing it's job. Equally, if the manifold and turbine housing get too hot they are susceptible to damage as are the bearings. Overfuelling just means that the materials don't exceed their recommended temperatures from the supplier.

The IEM is always flowing coolant, but that isn't so bad if you think about it. Hot exhaust gas running through the coolant jacket means the engine warms up faster so it runs more efficiently sooner. Coolant has very good thermal transfer properties so in normal running where exhaust temps are in the 400°C range, it's not really troubling the coolant jacket. The combustion chamber runs hotter. Ford's 3 cylinder Fox engine with the IEM runs Lambda 1 up to around 4000 rpm if I remember correctly and then overfuels after that point as an example.

Edited by zeppelin101 on Wednesday 19th March 10:23

conkerman

3,298 posts

134 months

Wednesday 19th March 2014
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To have any understanding of where vehicle and engine design is going you need to understand the homologation test cycles. The importance of these is absolutely prime, especially with fleet CO2 averages being imposed on the car manufacturers.

In Europe we use the NEDC.

This test cycle uses VERY gentle accelerations and the Extra Urban part of the cycle has a max speed of 120km/h again with pretty gentle accelerations. (from memory 2-3 mph/sec).

Downsized engines under there conditions are.

1) smaller so less fricton.
2) In a more efficient part of the speed load map over a larger engine using much smaller throttle openings.
3) be operating in closed loop for (probably) all the cycle.

So the engineers have used the characteristics of the test cycle to give the best possible results, In real situations, all bets are off as I certainly don't drive like that and the vehicle will be closed loop and not adding extra fuel to reduce EGT.

Edited as I missed out an important word....


Edited by conkerman on Wednesday 19th March 10:51

Luther Blisset

391 posts

131 months

Wednesday 19th March 2014
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Interesting scratchchin I always envisaged it creating huge stress on the cooling system while also slowing spool up. The main thing I didn't like about them is is that it makes manifold swaps impossible/a total ballache. I get the feeling the OEMs aren't too fussed. I wonder if they direct the flow out from the IEM straight into the heater matrix whistle

zeppelin101

724 posts

191 months

Wednesday 19th March 2014
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Luther Blisset said:
Interesting scratchchin I always envisaged it creating huge stress on the cooling system while also slowing spool up. The main thing I didn't like about them is is that it makes manifold swaps impossible/a total ballache. I get the feeling the OEMs aren't too fussed. I wonder if they direct the flow out from the IEM straight into the heater matrix whistle
Impossible to replace the manifold - it's integrated into the head casting. They are much less likely to be damaged anyway due to the cooler metal temps.

The designs that different OEMs employ for the manifold cooling jacket is quite interesting. Worth a google.

Any increase in lag due to an IEM is not really measurable in my experience. The boost threshold is a function of the turbine and compressor matching and at low speeds (<2000 rpm) the engine is typically not temperature limited so overfuelling is not required.

A couple of other posts in here have reiterated the same point about open loop fuelling which I have tried to address but seem to be getting ignored.

GrizzlyBear

1,072 posts

134 months

Wednesday 19th March 2014
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My cynical nature makes me think the move to small capacity turbos is all because cars are lasting too long, they have to chuck a few things that are fundamental to it working but cost a lot replace, so when it breaks at 7 or 8 year old it becomes uneconomic to repair and people scrap them and buy new ones.

nickfrog

20,871 posts

216 months

Wednesday 19th March 2014
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conkerman said:
To have any understanding of where vehicle and engine design is going you need to understand the homologation test cycles. The importance of these is absolutely prime, especially with fleet CO2 averages being imposed on the car manufacturers.

In Europe we use the NEDC.

This test cycle uses VERY gentle accelerations and the Extra Urban part of the cycle has a max speed of 120km/h again with pretty gentle accelerations. (from memory 2-3 mph/sec).

Downsized engines under there conditions are.

1) smaller so less fricton.
2) In a more efficient part of the speed load map over a larger engine using much smaller throttle openings.
3) be operating in closed loop for (probably) all the cycle.

So the engineers have used the characteristics of the test cycle to give the best possible results, In real situations, all bets are off as I certainly don't drive like that and the vehicle will be closed loop and not adding extra fuel to reduce EGT.
Exactly.

The move to turbo is strictly motivated by the results of the official cycle. Which doesn't necessarily have any bearing on real world usage and therefore real world mpg or emissions. There must be some correlation between the two of course but only in a particular driving style / usage.

I moved from a MINI Cooper S 1.6T to a much heavier 125i I6 3L NA engine.

Guess what, mpg is almost identical for the particular use I have of the car. If I did loads of town driving I reckon the BMW would have yielded worse results though.

Needless to say in terms of reliability, there is only one winner. And it ain't the MINI.

ORD

18,086 posts

126 months

Monday 1st September 2014
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Thread bump.

I have just done about 800 miles in a C180 over the weekend (a tc 1.6, I think), and its performance was pretty impressive at speeds above 40mph (below which it was chronic).

It was the most interventionist engine management system that I have ever encountered - helped by the God awful auto transmission, it was extremely reluctant to step outside of its most efficient operating parameters.

Still, it returned 47mpg or so over mostly motorway driving, which is quite impressive for a petrol engine in a medium sized car. I doubt it would do much better than a 6 pot in A and B road driving, though, given how highly stressed it had to be in order to extract performance.

Baryonyx

17,990 posts

158 months

Monday 1st September 2014
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I drove home from Glasgow the other day powered by a Jaguar 4.0 V8. It didn't use as much fuel as I thought it would, but more importantly, it felt fking great getting back behind a proper engine.

anonymous-user

53 months

Monday 1st September 2014
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Baryonyx said:
I drove home from Glasgow the other day powered by a Jaguar 4.0 V8. It didn't use as much fuel as I thought it would, but more importantly, it felt fking great getting back behind a proper engine.
There is a certain nice feeling about having a large capacity and lots of cylinders, but the problem is that the vast majority of car buyers don't care about such things. The majority of people care about running costs and CO2.

When I asked Mercedes why they didn't launch the new C Class with any 6 cylinder petrol or diesel engines, they said it's because they just don't sell many of them. Big engines will, increasingly, become the preserve of specialist cars, such as Merc AMG or BMW M Sport models, but as you can see from the A45 AMG, even that is changing.

I have just ordered a new BMW 428i, which has a turbocharged 2 litre. I would have liked the 435i, but the performance gain just didn't justify the increased costs, for me, at least.

delboy735

1,656 posts

201 months

Monday 1st September 2014
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GrizzlyBear said:
My cynical nature makes me think the move to small capacity turbos is all because cars are lasting too long, they have to chuck a few things that are fundamental to it working but cost a lot replace, so when it breaks at 7 or 8 year old it becomes uneconomic to repair and people scrap them and buy new ones.
I tend to be with you on this one. Not only that, small capacity big power surely produces a much more fragile engine. Likened to a top athlete,yes they do thing things us mere mortals don't, but they are prone to injury, and can then be out of action for months at a time....I fear this could be a future problem of "small" modern engines. I mean, a 1.0 engine with 125BHP...can't see them lasting the 275000 miles my old E34 managed. ( 2.5 straight 6 ).

Just as an aside, how many people on here genuinely check the CO2's when purchasing another vehicle ?? Have to admit, it's never been a consideration for me.........yet smile

J4CKO

41,284 posts

199 months

Monday 1st September 2014
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I don't think there is any reason why a well designed, small capacity turbo engine cant last indefinitely given proper maintenance and sympathetic usage.

Ford have put masses of development effort into the Ecoboost engine, turbocharged engines used to be a bit hit and miss but things have come on a lot, turbos nowadays are water cooled as well as being lubricated by the engine oil, the control aspects are much improved compared to the early days of the turbo and the engine is better monitored.

As long as owners change the oil, more critical with the temperatures generated inside a turbo and don't turn it off straight after hard usage it should be ok, but there is the potential pitfall I suppose, only time will tell.

The BMW Six cyl engine was mentioned as being a paragon of longevity, I do remember quite a lot of head warping/gasket issues.

jamieduff1981

8,022 posts

139 months

Monday 1st September 2014
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delboy735 said:
Just as an aside, how many people on here genuinely check the CO2's when purchasing another vehicle ?? Have to admit, it's never been a consideration for me.........yet smile
Me - never. Not once.

Of primary concern though, and the first filter to dismiss loads of chaff off the "What next" list is the available engine layouts and whether they make a nice sound - even in family cars.

It's vexing me that the only V10 production car I'm aware of that I like is the Viper. It's a sound track I'd love to own but all the European cars with V10s are either cosmetic mingers or have anti-enjoyment drivetrains.

I've had 5 V6s, but not a straight 6 yet smile A V12 is also on the must-get list smile

How efficient they are either in terms of CO2, BHP/litre, MPG or any of that crap - I really couldn't give a monkey's!