Which Engine Type Should Car Manufacturers and Buyers Choose?

[Ranger Mike]

thanks brewnog..the manufacturer of the WVO kit did a very nice job. this waste veggie oil kit is the only one on the market that can use partially hydronenated wvo. thisis the only kit that will work in the winter..in my opinion and those who researched the matter..it is estimated that fuel cost is about nine cents a gallon after conversion

 

[1bad90GT]

Turbo chargers take up a lot of space, and retain more heat in the engine compartment, but then the engine itself is smaller. A larger V8 is generally smoother (less vibration), and many prefer the sound and feel of a larger V8 (or even a V6). Torque versus rpm curver will generally be flatter with a non-turbo charged curve, more low rpm power.

It's hard to say which is really better. On the semi-high end, a Corvette Z06 uses a 7 liter V8 to make 505hp, while a Ford GT uses a supercharged 5.4 liter engine to make 550hp. The Corvette weighs 3150lbs, while the GT weighs 3485lbs. The Z06 gets 26 miles per gallon on the freeway, while the GT gets about 19.

The Z06 7.0 liter engine is light, lighter than the 480hp twin turbo 3.6 liter flat six used in a Porsche 911 Turbo.

its all aluminum with cross-drilled 6 bolt mains :)

 

[SonyAD]

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

Bigger cylinders have better volume to surface ratios. They also don't need to rev as hard for a given power output so pumping and reciprocating mass inertial losses can both be significantly lower, a big plus for efficiency.

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

So, any way you look at it, small engines really are for light duty.

Myself, I would choose a Citroën C5 3.0 HDi. But cost, taxes and insurance rates conspire against me so I still drive a Peugeot 2.0 HDi.

 

[xxChrisxx]

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

You can tune around the back pressure, that doesn't really make too much difference. The biggest efficiency killer is you have to run a lower CR in petrols, that's about it.

It's not too much of an issue in modern GDI engines and the turbo is the perfect mate for diesel (urgh devils fuel) as it increases specific power output and efficiency.

BTW: The C5 is a shocking car, more electonic gremlins than you can shake a stick at. It is one of the most comfotable rides I've felt though.

 

[brewnog]

You should note that turbocharging does not "sap efficiency" at all, it increases it.

 

[russ_watters]

Right - the reason a car with a turbo would tend to get a lower mpg than a car without one isn't the efficiency, it is the fact that it tends to run at a higher power output. But at constant speed on a highway, the one with the turbo will get lower mpg.

 

[SonyAD]

You should note that turbocharging does not "sap efficiency" at all, it increases it.

Turbocharging degrades specific fuel consumption. By way of exhaust restriction, like a fractured or molten cat. It takes work to pressurize the intake and it takes energy to do work. You may be getting more power but you pay for it in higher BSFC.

Right - the reason a car with a turbo would tend to get a lower mpg than a car without one isn't the efficiency, it is the fact that it tends to run at a higher power output. But at constant speed on a highway, the one with the turbo will get lower mpg.

The higher the boost you're running, the more efficiency drops. Not to say that the engine is otherwise running excellent efficiency, when off boost. The engine doesn't flow as well as a NA engine because the restriction posed by the compressor impeller, the turbo and the way the intake and exhaust manifolds are shaped isn't conducive to the best airflow.

Then again, the forced induction engine doesn't need to rev like a Honda to put out a given amount of power out of a given displacement. Which potentially helps greatly with pumping and inertial losses.

However, if you allow for increased displacement, there's not contest. Ceteris paribus, efficiency scales with cylinder size engine.

This is clearly evident by comparing the extremes. Ship engines and RC model engines, both two strokes. From the worst to the best specific power, from the best to the worst BSFC.

Despite supercharging being necessary for cylinder scavenging with the two stroke diesels. They can't run without supercharging. They are often turbocharged as well because they suxorz so bad with power.

You can tune around the back pressure, that doesn't really make too much difference. The biggest efficiency killer is you have to run a lower CR in petrols, that's about it.

You may also need to run richer.

It's not too much of an issue in modern GDI engines and the turbo is the perfect mate for diesel (urgh devils fuel) as it increases specific power output and efficiency.

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

BTW: The C5 is a shocking car, more electonic gremlins than you can shake a stick at. It is one of the most comfotable rides I've felt though.

At least it isn't German.

The French trying to make a German car or appeal to German car fanboys is an Oedipian tragedy.

 

[brewnog]

I disagree about an increase in BSFC, particulary "across the load range". Absolute fuel consumption would be debatable, but not brake specific. In my experience (on Diesel, but particularly spark ignition engines) fuel consumption at the matched points is notably superior with turbocharging.

 

[1bad90GT]

what does bsfc mean

 

[xxChrisxx]

Brake specific fuel consumption.

 

[SonyAD]

I disagree about an increase in BSFC, particulary "across the load range". Absolute fuel consumption would be debatable, but not brake specific. In my experience (on Diesel, but particularly spark ignition engines) fuel consumption at the matched points is notably superior with turbocharging.

I think you're comparing apples to oranges.

Naturally aspirated diesels are "indirect" injection designs and usually have comparatively rudimentary injection systems. They use Ricardo Comet swirl chambers in the head so they likely have lower volume to surface ratios and the associated heat loss.

Nevertheless, they tend to have higher to much higher CR than turbodiesels. The PSA DW8 (1.9D) workhorse, for instance, had a 23:1 compression ratio.

I would wager it has better WOT fuel consumption than the 1.4 HDi it was replaced by, which is turbocharged and has an 18:1 compression ratio, as the higher CR likely makes up for disparities in heat loss, fuel atomisation & cylinder pressure curve due to the HDi's multi stage, CR injection system. They both make near to 70 PS.

 

[russ_watters]

Btw, there was a typo in my previous post. I meant to say that at constant speed the mpg is HIGHER on a small engine with a turbo.

 

[russ_watters]

Also, the wiki on turbochargers clearly explains why a supercharger decreases efficiency while a turbo increases it: the super uses otherwise usable shaft energy to spin the compressor while the turbo uses otherwise wasted exhaust energy.

Also note that a gas turbine engine is much more efficient than an Otto or diesel and is basically just a tubocharger operaing on its own.

 

[SonyAD]

Not so. This is a very, very common misconception. A turbocharger restricts exhaust to get energy to pressurize the intake.

Exhaust brakes also restrict exhaust. Would you say exhaust brakes also use "otherwise wasted exhaust energy" since using them clearly does nothing in particular but has a retarding effect on the engine, by making it pump against, what is for it, essentially, a pressure gradient?

Another very common misconception is that turbochargers are powered, at least in part, by exhaust gas temperatures, again suggesting turbochargers have low or no parasitic losses.

You can't extract useful mechanical work directly from thermal agitation.

What you can do is contain the expansion of a working fluid under heating to increase pressure. Then extract useful mechanical work from the pressure gradient between the contained working fluid and the environment or a condenser, in the case of an external combustion engine.

So we're back to where we started. Boost isn't free as running on it increases backpressure, making the engine pump against, what is to it, a pressure gradient.

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

I had a big argument over this over at tdiclub.com before I was banned by some rightwing peasant with authoritah.

http://forums.tdiclub.com/showpost.php?p=2993984&postcount=147

 

[brewnog]

I think you're comparing apples to oranges.

Naturally aspirated diesels are "indirect" injection designs and usually have comparatively rudimentary injection systems. They use Ricardo Comet swirl chambers in the head so they likely have lower volume to surface ratios and the associated heat loss.

Nevertheless, they tend to have higher to much higher CR than turbodiesels. The PSA DW8 (1.9D) workhorse, for instance, had a 23:1 compression ratio.

I would wager it has better WOT fuel consumption than the 1.4 HDi it was replaced by, which is turbocharged and has an 18:1 compression ratio, as the higher CR likely makes up for disparities in heat loss, fuel atomisation & cylinder pressure curve due to the HDi's multi stage, CR injection system. They both make near to 70 PS.

I agree about the difficulty of making like-for-like comparisons, and by that token I'm not convinced by your comparison (the 1.4HDi was developed for relatively stringent emissions legislation, so spill timing is nominally far more retarded than had it been optimised for low SFC). These are tough comparisons to make.

 

[brewnog]

Btw, there was a typo in my previous post. I meant to say that at constant speed the mpg is HIGHER on a small engine with a turbo.

I thought so!

 

[xxChrisxx]

Not so. This is a very, very common misconception. A turbocharger restricts exhaust to get energy to pressurize the intake.

Exhaust brakes also restrict exhaust. Would you say exhaust brakes also use "otherwise wasted exhaust energy" since using them clearly does nothing in particular but has a retarding effect on the engine, by making it pump against, what is for it, essentially, a pressure gradient?

Another very common misconception is that turbochargers are powered, at least in part, by exhaust gas temperatures, again suggesting turbochargers have low or no parasitic losses.

You can't extract useful mechanical work directly from thermal agitation.

Noone apart from you seems to remotely think that. Exhaust is purely high enthalpy gas, that's all you are tapping into. The fact it's hot is just a matter of consequence.

So we're back to where we started. Boost isn't free as running on it increases backpressure, making the engine pump against, what is to it, a pressure gradient.

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

I had a big argument over this over at tdiclub.com before I was banned by some rightwing peasant with authoritah.

http://forums.tdiclub.com/showpost.php?p=2993984&postcount=147

Will you stop obsessing on picky **** for gods sake.

Everything is a compromise, overall turbocharging a passer vehichles engine yields a more suitable package for everyday driving. It's the whole reason for the industry trend of downsizing engines. Of course it has downsides that a NA car doesn't have, it also has many positives.

Yes enevitably exhaust gases are restricted, this is the whole reason why turbo cars have large downpipes and different exhaust headers. Overall the effect of the back pressure is largely negated. Back pressure is a bit of a misnomer anyway. This is the reason you can't just bolt on a turbo, as it will likely kill the exhuast valves without modification.

In a remotely modern injection diesels there is no downside to turbocharging as you aren't compressing fuel, so you don't have to drop the compression ratio. There is also no throttle plate so you don't get any pumping problems, meaning you also don't need bypass valves.

Overall turbochargers make a package that is far far more suitable for tooling around town in.Can I just ask, why do you think you are an immense authority on this subject? Frankly I can see why you got banned on other forums.

 

[russ_watters]

Sony: provide a reference to support your claims.

 

[SonyAD]

Which ones?

 

[brewnog]

Start with these...

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

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

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

Turbocharging degrades specific fuel consumption.

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

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

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

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

You may also need to run richer.

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

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

 

[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