Small engines vs. big engines?

[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.:tongue:
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.