Which Engine Type Should Car Manufacturers and Buyers Choose?

[darknyte]

If you were a car manufacturer, which of the following cars would you choose to manufacture, supposing you are only allowed to manufacture either kind:

1) a car with a small engine but with a turbocharger; or
2) a car with a big engine

Assume that both cars have other variables constant i.e. the same horsepower, the same acceleration speed etc.

Next, if you were a car buyer, which of the cars would you decide to buy?

 

[JasonRox]

I love small engines with turbochargers are on them, so I vote small one if I were a buyer.

Even if I were a manufacturer, I would make the smaller one because of the market is going with car buying.

 

[darknyte]

Are there any clear advantages of small engines with turbochargers, compared to big engines? Or any clear disadvantages?

I know that with most turbochargers, turbo lag might occur now and then, and that's something which will not happen for big engines. Any others?

 

[fleem]

Of course, there are a lot of variables that boil down to driver preference, but...

A simplified analysis suggests a small engine is better because it can run at higher RPM and thus move the same amount of fuel-air mixture through its cylinders as the big engine--which translates to power. Unfortunately, the size of the intake and exhaust valves become problematic. Understand that it isn't just a matter of figuring out a way to get the fuel-air through the cylinder (which is what a turbocharger solves, along with a moderate improvement in efficiency because of the higher cylinder pressure). It is a matter of how much power is wasted in the extra turbulence you get by really shoving that air through a smaller orifice. The turbocharger increases the fuel-air flow through the engine and increases efficiency via higher cylinder pressure, but it also takes power from the engine to spin and it increases turbulence of air through those little orifices.

 

[stewartcs]

Try searching this forum for "Turbos" or similar phrases. There is a lot of information on how them that might help answer your question.

 

[turbo]

Two words: Grand National

Turbocharged V-6 in a mid-sized Buick that could give 'Vettes a run for their money.

 

[rcgldr]

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.

 

[brewnog]

Turbo, definitely, though it depends entirely on application.

 

[Danger]

From the practical standpoint, as a manufacturer, I'd have to go with the smaller engine/turbo. As a consumer, not a chance. There's no replacement for displacement.

 

[Andre]

But the larger engine is likely to last longer. The turbo is boosted to stronger action, higher pressures, more tear and wear, limits the life span and boosts the maintenance bill.

 

[brewnog]

There's no reason that a turbocharged engine can't last far longer than a nat asp. Average life spans of a typical German turbodiesel are double; possibly triple that of a naturally aspirated engine of equivalent output. Just because an engine has been breathed on doesn't necessarily mean it's going to live less long.

Both configurations have their advantages; and these advantages are most prominent in completely different markets and applications.

There's no replacement for displacement.

I know there's some tongue-in-cheek there, but this is exactly the delusional view that just doesn't hold water any more. It's this myth that is largely preventing the North American market from seeing sense and buying (and preparing infrastructure for) 50mpg turbodiesels rather than 20mpg V8s, and arguably getting better performance.

I was having a conversation with an American colleague the other day, who when hearing that my car 'only' has a 2.4 litre engine, exclaimed "my lawnmower has more power than that!" Considering this guy works for an engine manufacturer, I couldn't believe that he was still equating displacement with power. He was amazed when I told him it develops nearly 200bhp and 380Nm of torque; and that I still get 45mpg.

 

[Stingray]

I know there's some tongue-in-cheek there, but this is exactly the delusional view that just doesn't hold water any more. It's this myth that is largely preventing the North American market from seeing sense and buying (and preparing infrastructure for) 50mpg turbodiesels rather than 20mpg V8s, and arguably getting better performance.

Can you give an example? I've never heard of anything that could approach that kind of efficiency.

 

[Danger]

it develops nearly 200bhp and 380Nm of torque

It'd make a good starter for a real motor.
I personally wouldn't own anything with less than 400hp, and over 1,000 is better.

Anyhow, the question was based from the manufacturer's standpoint, not the buyer's. While I might be wrong, I'm pretty sure that the costs would be lower for the little motor.

 

[darknyte]

wow guys, thanks for all the nuggets of information! they came in really useful!

 

[brewnog]

Can you give an example? I've never heard of anything that could approach that kind of efficiency.

50mpg?

VW's current 1.9TDI is quoted at 56.5mpg on a combined, 65.7mpg extra urban, in a Golf.
Audi's 2.0TDI A3 will do 51.4mpg combined.
Ford's 2.0 Duratorq TDCi will do 58.9 extra urban.

Just have a look in the back of any car magazine.

 

[Stingray]

50mpg?

No. You were claiming that a vehicle normally getting 20 mpg with a V8 could get 50 mpg with a turbodiesel while suffering minimal performance loss. While their efficiencies are very impressive, none of these examples fit that statement.

 

[brewnog]

No. You were claiming that a vehicle normally getting 20 mpg with a V8 could get 50 mpg with a turbodiesel while suffering minimal performance loss. While their efficiencies are very impressive, none of these examples fit that statement.

Ok, you could compare (say) a 3 litre Audi turbodiesel (155mph, 0-60 6.1 secs, 43mpg), with (say) a Cadillac STS V8 (155mpg, 0-60 6.2secs, 24mpg).

Probably not the best comparison, but it shows the point I was making.

 

[Stingray]

That's a nice example, although it looks like you're using the figures from an A4. The A6 is probably more comparable to an STS in size, and apparently gets 34 mpg with that engine.

Still, that's very good. I hadn't realized that diesels improved so much. My only experience in one was a Mercedes 240D from the 70's. That thing was dangerously slow on Los Angeles freeways

 

[brewnog]

Yeah that was the A4.

I think most petrolheads (myself included) are astounded the first time they drive a common rail diesel. My Alfa JTD even sounds like a straight six petrol engine, and pulls all the way to the red line with no perceivable turbo lag whatsoever. The only giveaways that it's a diesel are the 5,000rpm redline, the rattly cold start, and only having to fill up every 550 miles.

 

[1bad90GT]

Of course, there are a lot of variables that boil down to driver preference, but...

A simplified analysis suggests a small engine is better because it can run at higher RPM and thus move the same amount of fuel-air mixture through its cylinders as the big engine--which translates to power. Unfortunately, the size of the intake and exhaust valves become problematic. Understand that it isn't just a matter of figuring out a way to get the fuel-air through the cylinder (which is what a turbocharger solves, along with a moderate improvement in efficiency because of the higher cylinder pressure). It is a matter of how much power is wasted in the extra turbulence you get by really shoving that air through a smaller orifice. The turbocharger increases the fuel-air flow through the engine and increases efficiency via higher cylinder pressure, but it also takes power from the engine to spin and it increases turbulence of air through those little orifices.

actually small engines rev higher because of the overhead cam...but when you look a the 03 cobra which is dohc (has 4 cams) then rpm isn't an issue. and they come boosted from the factory which gives them 410 hp. u equipt it with just pulleys, intake, exhuast, and a tune, and you get 600 hp to the crank and about 500 to the wheels. i choose v8 over small engines any day. there's no replacement for displacement

 

[1bad90GT]

Ok, you could compare (say) a 3 litre Audi turbodiesel (155mph, 0-60 6.1 secs, 43mpg), with (say) a Cadillac STS V8 (155mpg, 0-60 6.2secs, 24mpg).

Probably not the best comparison, but it shows the point I was making.

unless your talking about the 2010 ctsv with 0-6 at 3.9 seconds with 556 hp. no cadillac sts or cts v8 came with less than 400 hp and slower than 4.9 seconds

 

[dr dodge]

ok, first off I vote large displacement.

now, if friction and losses go up with rpm, (more strokes/min) wouldn't heat, friction go down and efficiency go up with a slower rotating, high torque motor?
especially if you went from 6 speeds to 3. gearbox costs alone would provide savings, let alone increase of mechanical efficiency. to build it, it seems to me, actual costs would be less to a point with a bigger engine. with smaller and more precise construction needed, normal tolerances of manufacturer become larger as a percentage of the whole, making specialized machining requirements/tooling. Smaller and more precise mechanics require significantly more skilled labor to assemble/service.

The above discussions were good, but for the sake of this conversation, we need to keep diesel and gasoline seperate. So what then is the ratio of consumption/pounds payload moved/mile. locomotives get some pretty impressive numbers vs the turbo diesel in a VW, if you consider "total used power"/gallon of fuel

dr

 

[1bad90GT]

also smaller engines rev higher because the have a much smaller stroke. this can be a bad thing because the faster an object rotates the less torque it has

with that said...if you have 2 cars that weigh the same and a 4 cylinder tubro is against a v8 with same horse power rating (lets asume theyre both in the same car car) the v8 will win because of the amunt of toque advantage it has over the 4 cylinder. so the higher the 4 cylinder revs the less torque it will have from the line and it will have to really work to catch up at the end of let's say 1/4 mile and will be much harder

 

[turbo]

Bigger is not always better. A friend of mine has a bracket car, and has been national champion in his class. He had to work REALLY hard to make his 340 Duster competitive. When you look at the size differences in pistons, rods, etc between the 340 Mopar and the Chevy 283, you see why the small-block Chevy can rev up so quickly, and why those engines have been a favorite for drag racers for decades. Sometimes, you get a mix of qualities that make for a strong and robust engine. Going even smaller, Buick's little push-rod V-6 has proven to give a great power-to-weight ratio, normally aspirated, turboed, and super-charged.

 

[russ_watters]

Note the thread is 3 years old...

 

[xxChrisxx]

1. Noone, I mean no one needs the power output of a stupidly large displacement engine for everyday driving.

2. Arguing over who's engine has a large torque output is pointless in a racing capacity, as it's hooked up to a gearbox power output is all that matters. (The V8 would be more drivable and require less gears but that's not the point). You but that highly turbocharged 4pot engine hooked up to a DSG gearbox vs the V8 with a manual, the DSG will most likely win outright.

The advantage the V8 has means it requires less gearchanges to be in a certain % powerband. If you had 4pot manual vs V8 manual, the V8 has the edge. Being thilled by going in a straight like is... well meh. (top fuel is cool, but other drag racing is boring). On a real track, the 4pot would likely have the advantage due to having less inertia (aids in quickly speeding up and slowing down).

3. There is an industry trend for downsizing to smaller displacement turbo engines for a reason. They give you all the grunt of a larger engine when you need it, but at less fuel and emissions cost when you aren't using the power. (see point 1)It's only really the US that still insists on huge engines in non executive cars (becuase of the curious way your emissions standard are defined, it's easier to get a large displacement engine to pass), and frankly as far as engine technology is concerned you are all in the dark ages.

 

[brewnog]

I have to agree with Chris. I made some points in this thread about diesel engines a few years ago, but since then the developments have been huge. I dread to think of the trouble the oil companies would have trying to keep up with demand if any of you Americans ever actually drove a modern, high-output turbodiesel!

 

[Ranger Mike]

ouch..two year ago i dumped the Chrysler convertible for aVW beetle diesel 1.9 TDI and i get 44 mpg with automatic trans
my dodge cummins pickup is diesel
one huge consideration that has to be in the discussion is the government regulations to pas as the CAFE mileage requirements and epa smog regs...these are what doomed the big block V8 engines in 1972..and will eventually kill off the balance of V8 technology...
btw i goot convert the VW to run on waste veggie oil quick before the stuff hits 5 dollars a gallon

 

[brewnog]

Good luck with the veg oil conversion, but you might struggle with that PD engine if you have anything other than warm winters. 5 dollars a gallon sounds great though!

 

[lifeattthesha]

I would go electric!

 

[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.