Which Engine Type Should Car Manufacturers and Buyers Choose?

[xxChrisxx]

To be fair, in specific instances a lot of what he said is true and can be backed up. But they are all wooly statements that are totally irrelevant to the points being made by other people.

Such as in petrol turbo cars: "you may have to run richer" is true for very highly boosted cars . Yet in most road going instances you never have to run rich, but generally the CR is lowered slightly.

So although the statement itsself is true, in context it's just ********.

Also the statement hat it increases back pressure. This is also true, but again in contact it's irrelevant as the exhuast system is resigned to reduce the impact of this inherent phenomenon. Pretty much to the point that it can be ignored for passenger cars. (It may become relevant again if you are running a race prepped engine)He also picked out negative traits for petrol turbos (such as increased pumping loss), and tried applying them to things when I was specifically talking about diesels. Especially at part thottle, where petrol turbos are at their worst, but diesels are at their best.
EDIT: Also he keeps banging on about how it increases BSFC across the range... well of course it does. They are making more power across the entire range, with more power being made you'd expect more fuel to be burnt.

 

[SonyAD]

Small capacity turboed is more efficient for mostly low load applications.

When you're not running on boost, yes.

Forced induction is not a free lunch.

Big capacity aspirated is better for mostly high load applications (near peak power most of the time and tracing the torque graph when not).

When you'd be running on boost and at higher rpm if you had a small displacement turboed engine, yes.

Forced induction is not a free lunch.

Turbocharging, not just supercharging, also saps efficiency by increasing pumping losses through extra back pressure.

I've already backed this up. See comparisons with exhaust brakes and other restrictions, like catalytic converters, particulate traps, etc.

Why do people seem to think that pumping against a pressure gradient doesn't require energy.

Turbocharging degrades specific fuel consumption.

Everybody seems to agree that supercharging increases fuel consumption / power but turbocharging is believed to be a free lunch.

Why not fit the biggest turbo you can find to a 1.2 tdi and see how that helps specific fuel consumption at WOT.

You may be getting more power but you pay for it in higher BSFC.

See above. Even http://en.wikipedia.org/wiki/Turbocharger#Comparison_to_supercharging" that the marginal increase in power is less than the marginal increase in fuel consumption, with supercharging. Yet this is in the context of turbocharging being a free lunch in comparison, you see.

In reality, you are still getting the energy needed to pressurize the intake from the engine. It's just that you get it differently. Instead of a mechanical coupling (belt, pulleys) you use fluid coupling (turbo in the exhaust stream, an obstruction which increases backpressure which takes torque away from the engine on the exhaust stroke).

A far leaner and flexible system, which scales boost with load as needed instead of scaling with engine rpm and requiring a gearbox or a bypass and/or overboost release valve.

But you still have to burn more fuel for the same amount of power you generate under WOT than a bigger, naturally aspirated engine of the same compression ratio would make without boost.

And most likely need to rev higher as well, opening another can of worms with reciprocating mass inertial losses and pumping losses, which exhibit squared growth with engine rpm.

The higher the boost you're running, the more efficiency drops.

Yup. Because more energy is required to maintain higher boost. Energy that isn't going, more or less, directly into propelling the vehicle but simply pumping more air into the cylinders on the intake stroke.

the way the intake and exhaust manifolds are shaped isn't conducive to the best airflow.

Yup. Have you seen the exhaust manifold on an Opel Zafira 2.2 cdti? It's a T-pipe. Like something right out of the 1920s. Haut couture on a model T.

{Two stroke engines}can't run without supercharging. They are often turbocharged as well because they suxorz so bad with power.

That's not an accurate quote. I was talking about marine & industrial app. two stroke diesels.

They aren't self charged for intake and cylinder scavenging as they can't run the intake charge through the crankcase because of the risk of a runaway on oil fumes. Being compression ignition engines and all.

Without supercharging they wouldn't run at all. Sometimes they are turbocharged in addition to supercharging.

You may also need to run richer.

In a turbo petrol application, yes. You may also need to retard ignition advance to the point that much of the power gain is lost.

Turbocharging doesn't increase efficiency. It increases BSFC across the load range.

Aye. Even when not running boost, the compressor's impeller and turbo are just another impediment to pumping air through the engine and back out to the atmosphere as easily as possible.

The turbo is just another obstruction laying around in the exhaust stream close to the engine and the intake and exhaust manifolds are often rudimentary, cast iron for exhaust and aluminum for intake, with no great care having been taken in the design of the runners to optimise flow as would be the case with decent NA petrol engines. Most turbodiesel 4 cyls. are 2 valves per cyl. as well.

Now it might not be a significant difference but still.

the marginal increase in fuel consumption under boost is greater than the marginal increase in torque at a given rpm.

See above.

EDIT: Also he keeps banging on about how it increases BSFC across the range... well of course it does. They are making more power across the entire range, with more power being made you'd expect more fuel to be burnt.

Note that BSFC is rate at which fuel is being consumed over the power produced.

 

[xxChrisxx]

Note that BSFC is rate at which fuel is being consumed over the power produced.

So. This means nothing by itsself as it doesn't take into account the 'potential' of the engine. You are producing more power, using slightly more fuel to do it, actual fuel usage for a spcific power output is generally reduced compared to a NA engine. This is reflected in vastly higher BMEP values for turbochrged engines.

The point is no one is disputing that for a PETROL engine. Thermal efficiency decreases with the use of a turbo charger. The point is the volumetric efficiency vastly increases. This increase in volumetric efficiency outweighs the loss of thermal efficiency.

So as an analogy: you get slighty less bang for your buck, but you use far less bangs to ge the job done. Net result: you spend less for the same job.

This is the whole concept of downsizing. So where as a 2L petrol engine may be using 2000 revs to achieve a power output (putting it lower on the BSFC map) a 1.4L Turbo will produce the same power at a more favourable point say 3000rpm.

Which is why all the talk of BSFC, in the context in which you are using is pointless.

Everybody seems to agree that supercharging increases fuel consumption / power but turbocharging is believed to be a free lunch.

Why not fit the biggest turbo you can find to a 1.2 tdi and see how that helps specific fuel consumption at WOT.

Ok I am going to size this up so you can read it.NOONE IS SAYING THAT IT IS A FREE LUNCH ONLY YOU ARE

Supercharging takes power directly from the engine. Turbocharging doesn't, you get the air compression essentially free. That is what people are saying. So you may lose 2 or 3 HP due to back pressure, but you will be getting 10-20hp worth of air compression from the exhaust. That is a net gain when you dump fuel in and get 50hp (numbers are just plucked form thin air, but seem semi sensible).I don't know if there is a language barrier here, but you are arguing the toss about things that we already know and have said.

You are also bitterly disagreeing with two mechanical engineers (iirc brewnog's job is acutally doing something with engines) and I specialised in motorsports engineering.

Everything you've said is true in itesself, but totally irrelevant in the grand scheme of things.

 

[Ranger Mike]

I stand with Turbo-1, Brewnog, Danger, Dr. Dodge and xxChrisxx..these guys know!

 

[jack action]

I did some calculations to raise the level of the discussion.

I compared simple P-V diagrams to find out the thermal efficiency (no friction losses) of different engine configurations. Thermal eff is directly linked to BSFC.

The reference NA engine has CR = 10:1, an inlet temperature (T_in) = 288 K and the temp at the end of the compression stroke (T_comp) = 645 K.

Then I compared SC and TC engines with the same CR at a PR of 1.5 and 2 (7.35 psi & 14.7 psi boost), T_in becomes 330 K and T_comp = 738 K.

The previous might be good for CI, but for SI - if we assume the NA engine is at the knocking limit of the fuel - we have to adjust the CR of the SC & TC engines to get T_comp = 645 K. The new CR was 6.8 for PR = 1.5 and 5.2 for PR = 2.

If we put an 80% efficient intercooler after the compressor, we can raise the CR to 9.2 for PR= 1.5 (8.6 for PR = 2) without going over T_comp = 645 K.

On the picture, I have put an example of the simplified P-V diagram (log-log) for a TC intercooled engine and the results for the thermal efficiencies.

I think I didn't make any mistakes.

With a SC, eff goes down, no matter what. The higher the PR, the lower the eff.

With a TC, the eff goes up with PR, only if we keep the same CR. As soon as we lower the CR, eff goes down just like the SC engine, while being slightly higher.

With a low PR, a TC and an intercooler, the losses are minimal.

What do you think?

 

[xxChrisxx]

Blimey Jack you are keen as mustard. All the numbers you came up with look about right (I know they were all assumed but it's surprising how close they are to real engines).

The VW 1.8 20V engine in the Golf Gti comes in a TC and non TC.

Non TC - 125BHP output CR - 10.5:1. Peaky engine.
TC intercooled - 150BHP CR - about 9.3:1 iirc. Nice flat torque curve. (the one I've got :D)

So those numbers are pretty damn close. I don't think they can be argued with.

 

[jack action]

The problem with using data from real engines (especially when they come from publicity) is that they don't always give the full picture. For example, a manufacturer could build a sport car by putting a turbo on an engine from their family car, but if you read the owner's manual, you find out that it also run on 92 octane instead of 87, alloying a higher compression ratio, thus higher efficiency. In such a case, comparing both engines becomes pointless.

We can argue forever by always finding an engine that contradicts or supports whatever we want when we only look at what serves our purpose.

Thermodynamic seems to be a very useful tool in such a discussion. After all, it's a physics forum .

 

[anonimus]

Hi there everyone.
New to this forum. Read all this discussion. OK, there are pros and cons to both types of engines.

How about from fuel consumption point? And let's be more realistic.

As I see it - 1,6 NA engine, simple 4 cylinder, you have to rev higher to drive at 100 km/h and if car is full of passangers, so you use more fuel.
At the same time larger engine, some 2-3 or 4 liter - you get good power output and torque at lower RPM, and can cruise on freeway at lower RPM.

Diesel is completely different story and it needs turbo to work good, but in gasoline engine with turbo and smaller displacement - you also need to rev higher to get power out of the engine and this means more cycles, more fuel to my understanding.

Am I wrong / right?

 

[xxChrisxx]

The idea was small capacity turbo vs large capacity naturally aspirated. Not small N/A vs large N/A.

 

[anonimus]

Did not expect answer this soon.

OK, so be it, what is the comparison from fuel consumption point of view?

small capacity turbo vs larger capacity naturally aspirated

 

[xxChrisxx]

A smaller turbo of equal power to a larger capacity N/A will always be better on fuel overall under normal use. This is the whole reason why the car industry is downsizing to smaller capacity turbos.

Turbos allow you to have the fuel usage of a small engine when you don't need to get somewhere quickly, but the power of a large engine when you need it.

Turbo engines are less efficient than N/A becuase they suffer from increased pumping loses at part throttle and a lower compression ratio. However this is more than made up for by the savings of using a smaller engine.The only time that you'd start to get larger engines being better on fuel was if you were running a significant amount of time at full throttle. As normal cars spend most of their time pootling around town not using any power, a smaller engine is more suitable.

 

[brewnog]

Chris's post is spot on.

 

[Topher925]

If I was a manufacturer I would put a larger NA engine in all my cars to reduce cost and achieve a higher average fuel economy.

I would never buy a car with a turbo in it unless I was having a mid-life crisis. I'm not a big fan of fast cars and prefer something with good mileage and low emissions. Plus, turbo-ed engines suck in the cold.

 

[Topher925]

A smaller turbo of equal power to a larger capacity N/A will always be better on fuel overall under normal use.

Do you have anything to back this up? There's good reasons why the most fuel efficient gasoline fuel vehicles don't have turbo chargers.

Turbo's always reduce engine efficiency, ALWAYS. And unless you're making engines out of pig iron, saving a little displacement (at the expense of compression ratio to boot) isn't going to have that big of an impact on weight savings.

Supercharging takes power directly from the engine. Turbocharging doesn't, you get the air compression essentially free.

NO! This is a common misconception. The only advantages turbo's can have over super chargers in terms of efficiency is from near adiabatic expansion of exhaust gases (and that ain't much) and no losses from a belt or drive train. There are also a lot of "ifs" that come from compressor design and so forth. For example, a centrifugal compressor is much more efficient than a screw type compressor. Often a reason for turbo efficiency being greater than super charger efficiency is just that the impeller has a better design and is more efficient due to a much higher rpm. Its tough to build a gear train that can spin an impeller at 100K+ rpms.

 

[xxChrisxx]

Do you have anything to back this up? There's good reasons why the most fuel efficient gasoline fuel vehicles don't have turbo chargers.

The reason that is becuase American car manufacturers have been horribly backwards with engine design. American manufacturers like cheap, with no regard to environmental cost. Turbocharged cars are complex and therefore are disregarded. They are stuck in a rut, American consumers hove your attitude to turbo motors, so manufacturer will make them for fear of not selling. The consumer can't buy them if no one sells a good turbo motor.

This however is changing. Get with the times. Engine downsizing is the trend within the industry to provide the power and meet emissions standard , you will find it in the vast majority of cars world wide. If not this generation, then the next generation of cars.Ford is putting a 2.0L Turbo over it's V6 in the new Explorer. Similar power output with a boost in economy of aprox. 30%.

VW are replacing pretty much it's entire line with reduced capacity turbos. With the most extreme being the 1.4TSI putting out 150bhp. Audi as part of that group are doing it
http://green.autoblog.com/2010/04/0...ead-engine-downsizing-to-increase-fuel-econo/

Toyota are doing it
http://paultan.org/2010/11/24/toyot...injection-route-for-next-generation-vehicles/

Volvo are doing it
http://green.autoblog.com/2010/12/0...downsized-engines-in-2013-ponders-diesel-for/

Even Bentley are doing it in the next Continental GT.

http://www.frost.com/prod/servlet/market-insight-top.pag?docid=195091644
Market insight from a consultancy website regarding turbos and downsizing to meet emissions standards.

http://www.google.co.uk/url?sa=t&so...sg=AFQjCNGI2CealQIuJ63ZPLZCxt-eFTqUwQ&cad=rja
A sort of technical pamphlet showing downsizing.

As I'm no longer at Uni I don't have access to the SAE papers archive. However there are a whole host of papers looking at cool stuff like variable vane turbos and direct injection to aid in downsizing.

http://papers.sae.org/2009-01-1472
http://papers.sae.org/2009-01-1053

I could go on, you can search the site on your own. There zillions of papers regarding this subject.

EDIT: You are doing a PhD iirc, your Uni is likely to have a subscription to the SAE paper database.

NO! This is a common misconception. The only advantages turbo's can have over super chargers in terms of efficiency is from near adiabatic expansion of exhaust gases (and that ain't much) and no losses from a belt or drive train. There are also a lot of "ifs" that come from compressor design and so forth. For example, a centrifugal compressor is much more efficient than a screw type compressor. Often a reason for turbo efficiency being greater than super charger efficiency is just that the impeller has a better design and is more efficient due to a much higher rpm. Its tough to build a gear train that can spin an impeller at 100K+ rpms.

The advantage comes from the fact that you are running the turbo (ie compressing the air) with something you would otherwise just throw straight out the exhaust and not something you want to use. It's like adding an LP turbine to a power plant to take advantage of the energy still left.

I was talking about system efficiency not component efficiency.

I would never buy a car with a turbo in it unless I was having a mid-life crisis. I'm not a big fan of fast cars and prefer something with good mileage and low emissions. Plus, turbo-ed engines suck in the cold.

Not only that, turbo cars a far easier to drive. As they have fat torque from the minute they come on boost. Leading to a wider power band.
http://autospeed.com/cms/title_Turbod-for-Fuel-Economy/A_109931/article.html

Some of the things in this link are a bit overly basic, but it gives a good indication of how a turbo alters the power band and how downsizing an engine but using a turbo to match performance works.

If you want something with good mpg and emissions, it's time to get yourself a new downsized turbo then isn't it.

 

[brewnog]

I attended an Engine Downsizing conference recently. I obviously can't regurgitate every detail, but we looked at some fabulous case studies. The one which stuck in my mind was taking a 5.0L naturally aspirated V8 automotive gasoline engine, and replacing it with a single stage turbocharged 2.0L I4. While it's easy to match peak power and peak torque, the interesting bit was that they'd matched the entire torque curve of the V8. Going from low idle to full load at 1,000rpm was something like 0.25 seconds, which they've shown to be imperceivable in practice.

Given that the baseline was using direct injection, the effective downsize was extraordinary, and this was just using a single stage turbocharger. Some of the case studies we saw using twin stage turbocharging were incredible (>35 bar BMEP, <230g/kWh).

The reason manufacturers have traditionally favoured larger, lower rated engines is because they haven't had the legislative CO2 drivers which are now coming through thick and fast; and that fuel consumption hasn't been as big a challenge as it is now. Those days are gone in Europe, and I can't see that North America can hold off for long. Downsizing is here to stay.

 

[anonimus]

So I see the solution for normal comfortable driving and fuel economy at the same time as car with reasonable size engine - like about 2.0L-2.5L with turbo.

 

[xxChrisxx]

The one which stuck in my mind was taking a 5.0L naturally aspirated V8 automotive gasoline engine, and replacing it with a single stage turbocharged 2.0L I4. While it's easy to match peak power and peak torque, the interesting bit was that they'd matched the entire torque curve of the V8.

Some of the case studies we saw using twin stage turbocharging were incredible (>35 bar BMEP, <230g/kWh).

That is incredible. I wouldn't have thought it possible to match the full curve with a single stage. It'd be interesting to see how it performs over a lifetime though, the increased loading must be huge.

 

[anonimus]

I forgot to mention that money is the issue, if I could buy new car with efficient turbo engine, I would not have to consider all pros and cons.

How about 10 years and older cars, I am guessing, that turbos were not that good.
I of course will look for direct injection, do not know about diesel, about -20 -30*C at winter sometimes..
To my understanding bigger displacement older engine had less stress on it and is better preserved, or isn't it?
Yes and fuel economy is still the subject.

 

[brewnog]

That is incredible. I wouldn't have thought it possible to match the full curve with a single stage. It'd be interesting to see how it performs over a lifetime though, the increased loading must be huge.

Funnily, that question was asked. While no durability testing has been done, peak cylinder pressures are held within the limits seen by a light duty diesel engine of comparable displacement. Obviously there will be concerns, but they didn't seem to expect anything which would inhibit the proposal's viability.

 

[Topher925]

The reason that is becuase American car manufacturers have been horribly backwards with engine design. American manufacturers like cheap, with no regard to environmental cost.

One of the links you posted said its only the European manufacturers that use the turbo strategy. Every American AND Asian manufacturer currently are taking the NA route in general. And I wouldn't say American manufacturers are backward at all. The Ford EcoBoost is a fantastic engine that uses state of the art technology including a turbo. Its just very expensive and the NOx emissions can be high.

Turbocharged cars are complex and therefore are disregarded. They are stuck in a rut, American consumers hove your attitude to turbo motors, so manufacturer will make them for fear of not selling. The consumer can't buy them if no one sells a good turbo motor.

Engines in general are complex, with or without turbos. And I don't think most consumers are afraid of turbos. I always thought that was a big selling point for Saab's was that most of their models could come with a turbo.

This however is changing. Get with the times. Engine downsizing is the trend within the industry to provide the power and meet emissions standard , you will find it in the vast majority of cars world wide. If not this generation, then the next generation of cars.

I'm hoping the next generation of cars won't have turbo's because they won't have ICEs.

Ford is putting a 2.0L Turbo over it's V6 in the new Explorer. Similar power output with a boost in economy of aprox. 30%...

Yes, all very good examples, especially the Ford Ecoboost engine. However, you're not comparing apples to apples. All the engines in the links you posted include a lot more than turbo's to aid in fuel efficiency. I believe just about all "downsized" engines that use turbos also use direct injection or DISI which in itself accounts for huge efficiency improvements, I would argue much more than a turbo.

I think engine downsizing is going to occur which will include the introduction of turbos and direct inject into cars. Both technologies compliment each other extremely well. But I didn't think that's what we are talking about? I thought this thread was about more classical engine tech that used port injection like your average car. If this isn't the case, then you should also be comparing NA engines that use the Atkins cycle, variable valve technology, and direct inject.

The advantage comes from the fact that you are running the turbo (ie compressing the air) with something you would otherwise just throw straight out the exhaust and not something you want to use. It's like adding an LP turbine to a power plant to take advantage of the energy still left.

No it isn't. A turbo doesn't use any wasted power with the exception of power that is extracted from exhaust gas expansion. When you add a turbo to an engine, the pistons have a greater exhaust gas pressure due to the obstruction that the turbo creates. In other words, the engine has to work harder to push out the combustion products on the exhaust stroke. This is by no means "free" power which a lot of people like to believe.

The power extracted from the thermal expansion of exhaust gases can be determined by measuring the temperature difference before and after the turbo. However, heat transfer needs to be taken into account as well. That impeller spinning at over 100K rpm has a very very good convective heat transfer coefficient.

Not only that, turbo cars a far easier to drive. As they have fat torque from the minute they come on boost. Leading to a wider power band.
http://autospeed.com/cms/title_Turbod-for-Fuel-Economy/A_109931/article.html

I would say turbo lag makes them more difficult to drive but to each his own. I'm not a race car drive, so such things don't really matter to me when choosing a car.

If you want something with good mpg and emissions, it's time to get yourself a new downsized turbo then isn't it.

My 05 Civic is rated for 26/35mpg and is classified as a ULEV. What downsized turbo ICE that's available in the US can provide the same emissions and fuel economy for around the same price?

 

[Topher925]

Funnily, that question was asked. While no durability testing has been done, peak cylinder pressures are held within the limits seen by a light duty diesel engine of comparable displacement. Obviously there will be concerns, but they didn't seem to expect anything which would inhibit the proposal's viability.

I think its important to keep in mind that just because something is viable, doesn't make it practical or even commercially acceptable. Viably, I could build a car that will out run and out perform a "downsized" ICE with the only emissions being 100% pure H2O. That doesn't mean its practical however. Well...not yet anyway.

 

[xxChrisxx]

It's called progress. Time will tell if what your or I say is right or wrong.

You asked for examples of downsized engines giving better mpg vs performance and I gave some. I really can't be bothered responding with increasingly long posts to a moving topic purely for a sake of argument.

Also actually read the links. The one with the torque curve shows a direct comaprison of a TSI engine (Vw direct injection) to the same TSI with a turbo.

edit: I don't mind discussing something, it's not to dismiss what you are saying. It's just the posts and number of topics being discussed are growing thick and fast. It makes having a meaningful chat about it quite difficult.

 

[jefswat]

I frankly don't have the faith to buy a turbo charged engine from any manufactures currently. Its one thing is they turbo charge a 6+L diesel engine, but its quite another to turbo a small engine. Yeah sure when everything is working great the car and engine is amazing, but if anything is out of whack it can cause big problems. NA cars are easier to trouble shoot since there is less going on. Not to mention, most car companies now like to cut corners. I don't want anyone cutting corners on a part that can shred little pieces of metal and suck them into my engine. I know a lot of mechanical engineers, and I'm afraid to touch anything they've designed. Some of the things people do I swear. I also know a significant number of structural engineers like that...

Bottom line: large displacement engine's are not very efficient in many respects. Turbo charged engines are more expensive, complex, and overall pains to work on. I would be more comfortable owning a NA for my DD, but I'd love either for a weekend car.

 

[HowlerMonkey]

The big reason you don't see bigger gains from the turbo diesels is that most are shaped like phone booths.

Put a turbodiesel in an aerodynamic sports car and watch the mileage figures go up.

 

[fredric21]

The smaller engine would have to win every time. Because the parts are smaller, raw material costs are lower. Smaller bores and strokes mean less friction and less total area for heat flux into the coolant.
The entire engine will be lighter and more responsive due to less part inertia. Higher rpm's are possible with no loss of reliability because piston speeds would be similar to the larger engine.
As an example, I have designed an engine with only two cylinders for 500cc's displacement and a theoretical 406 BHP at 15,000rpm.
I am currently assembling the prototype in my garage. It has both a supercharger and a turbo for BMEP's up to 620psia using water/methanol injection to mimic 150 octane gasoline.