Why do different engine configurations (V6, I6, V8, etc.) have different power characteristics?

It is common knowledge, in automotive circles, that different engine configurations will have different power delivery characteristics. I’m not talking about vibration, sound, or anything like that, but essentially the shape of the torque/power curves.

For instance, my old BMW had an 2.5L inline-six engine. This engine produced very little power at low RPMs, and thus had a fairly “peaky” torque curve. In order to get good performance, you had to operate the engine somewhere between 3000 to 4500 RPM. Performance was fairly marginal outside of that range.

My new Jeep has a 5.7L “Hemi” V8. It has a much flatter torque curve, and produces much more power lower in the RPM range than the BMW (even accounting for the Jeep’s much higher overall power output).

What specific characteristics of an engine configuration affect the shape of torque/power curves, and why? I assume it has something to do with the bore/stroke ratios typically used for each engine configuration. Does that mean you could construct an inline-6 with the same low-end “grunt” of a typical V8, if you simply designed it with the same bore/stroke ratio used for a typical V8?

Really? I have never heard of such a thing. If this was common knowledge why are you unable to explain it?

What you describe doesn’t necessarily have anything to do with the engine configuration. The BMW engine is smaller, is OHC and might be more performance oriented as it sits in a lighter car, so obviously it would perform as you describe, a Cummings diesel I6 is not going to be the same even though it is of the same configuration.

The only thing that might be an issue is the fact that crossplane V8s inherently have more rotating mass in the crank, for balance, than a flat plane V8, and so would not be as responsive as a flat plane V8.

You’re starting from a faulty premise. Just because BMW designs and tunes their straight sixes for revvy top-end power doesn’t mean that’s inherent to a straight six. The Chrylser slant-six was not a high-revving engine, nor was the Ford 300 straight six. And there’s no way you can call the big straight six turbo diesels in almost every semi truck “high-revving.” what defines how an engine’s powerband will look depends on a host of factors e.g. bore/stroke, cam profile and timing, airflow. Now it’s more likely that a smaller four or six cylinder will have a powerband higher in the RPM range because they have to make up for a lack of displacement by revving higher ( hp=torque*RPM/5252.)

It also has over twice the displacement.

Again, it’s mostly about the specific design choices used in the engine, and not the cylinder layout. And yes, you can easily create a straight six with a lot of low end torque. There’s one under the hood of pretty much any vehicle branded “Kenworth,” “Freightliner,” and so on. Hell, back in the '90s Ford pickups had a 300 cu. in. six as the base engine which had slightly lower torque/hp ratings than the 302 cu. in. V8, yet the six made its power lower in revs.

Stroke length dictates mechanical limits on RPM for a couple of reasons:

  1. long stroke + high RPM = high piston speeds, which generates a lot of heat where the piston and its rings slide on the bore. Pick any car, check the redline on the tachometer, and divide by the engine’s stroke length; you’ll come up with a number comparable to most other cars. High-performance sports cars push this limit a bit, but not by much.

  2. long stroke + high RPM = high piston accelerations. at TDC, as the piston stops and begins moving downward, the piston ring can lift up off of the land if combustion chamber pressure isn’t enough to hold it down. The result is more hot combustion gases blowing by the piston ring, which shortens the life of parts.

Check any high-revving engine, and you’ll find a short stroke length. Diesels, which typically aren’t very high-revving (because more time is required for diesel combustion) tend to be designed with longer strokes. Extremely large marine diesel engines, with stroke lengths measured in feet, usually don’t rev to more than a few hundred RPM. OTOH, the Yamaha R6 motorcycle, which has a stroke of just 42.5 mm, redlines at somewhere between 16,000 and 17,000 RPM.

If you’ve got short stroke length, and you want any kind of displacement, you’re going to need large bores, or a lot of pistons, or both. This is why Formula One race cars have V-8 (and until just a few years ago, V-10) engines, despite having displacements of 2.4 - 3.0 liters; compare that with my Nissan Maxima, which spreads 3.5 liters of displacement over just six cylinders. The downside is that my Maxima will not rev to 19,000 RPM. :smiley:

The power curve for an engine depends almost entirely on how it breathes. Exhaust system characteristics matter for this, but not nearly as much as intake system characteristics. Most performance-oriented gasoline engines these days use four valves per cylinder - two for exhaust, two for intake. That provides the opportunity to maximize airflow, even at elevated RPM. To take advantage of that opportunity, designers use the inertia of the air in the intake system, and the fact that air is compressible. Want an engine to make maximum torque at some arbitrary RPM? Simple: set up the intake system so that at that RPM, it stuffs as much air into the cylinder as possible before the intake valve closes. It’s a bit like tuning the pipes in a pipe organ.

Suppose you want an engine to produce lots of torque at low RPM. you make the air duct leading from the airbox to that cylinder skinny and long, and you set the intake valve timing so that it closes relatively close to BDC. At mid-stroke, the air is flowing very fast through the duct; it’s got lots of inertia. When the piston reaches BDC, that momentum continues cramming air into the cylinder. If you’ve designed things correctly, that air flow comes to a stop just as the intake valve closes and traps the maximum amount possible in the cylinder. Presto, your engine is making peak torque at this RPM. What happens if the RPM increases away from this ideal? Now the piston passes through mid-stroke too quickly, so the air in the duct doesn’t have a chance to accelerate to the desired maximum speed; moreover, the intake valves close too soon, before the air has come to a dead stop, so you haven’t crammed as much air into the cylinder as you were at lower RPM. Presto, the engine is now making less-than-peak torque.

Want an engine to make peak torque at higher RPM? shorten the intake tract a bit so those pressure spikes arrive at the intake valve sooner, and leave the intake valve open a little longer, even after BDC. Seems paradoxical, but even after BDC when you would expect the piston to be pumping air backward into the intake system through that open valve, the momentum of air in the intake system is still shoving even more air into the cylinder.

Want an engine to make peak torque over a wider range of RPM? Use variable valve timing. In the ideal, at higher RPM you would open the intake valve earlier (before TDC) and close it later (after BDC). A lot of production passenger cars simply shift the phasing of the camshaft so that the intake valve opens and closes later; this is less than ideal, but it’s economical. If you’ve got more money, you can also change the geometry of the intake system. The cheap way is to keep one of the two intake ports closed off at low RPM, and open it at higher RPM; there are a number of production passenger cars that do this now. The expensive way is to actually make those intake ducts adjustable in length: some late-model high-end sportscars (Ferrari, etc.) utilize computer-controlled intake ducts that extend or retract into an airbox according to RPM so that they provide very optimum breathing over a huge RPM range.

Different vehicles have different needs, and the engine’s characteristics are designed into them so as to best meet those needs. An 80,000-pound tractor-trailer needs an engine that can deliver lots of torque at very low RPM; without that, the driver will fry the clutch just trying to get started from a dead stop. They don’t need a shitload of power, though; most big-rigs have a few hundred peak horsepower, not much considering their weight. In other words, they don’t make much torque at high RPM. These are the characteristics you’re likely to find on any vehicle designed for towing applications.

Drag-racing cars have similar needs: they want a lot of torque to launch quickly off of the line.

Track racing cars don’t need low-end torque for zero-speed launches; instead, they need lots of power to accelerate quickly and maintain high speeds even in the presence of heavy aerodynamic drag. So they deliver their peak torque at high RPM, or the spend money on a variable-geometry intake system as described above.

Run-of-the-mill production passenger vehicles tend to be tuned with a fairly flat torque curve - not extremely peaky at either high or low RPM. They’re designed to be easy to drive, rather than being high-performance.

Thank you for this (and the rest of that post). It’s always boggled me why Toyta gave the MR2 next to no low-end torque. It’s unimpressive from a dead stop but if you keep it between 4000-8500 RPM (well above the point where you start wanting to shift in most other cars) it’s a lot of fun. It suits the cars other qualities–such as being nimble enough that, once you get it up to speed, you don’t really need to slow down for curves.

No g in Cummins. Pedantic I know, just a pet peeve of mine.

http://cumminsengines.com/every/everytime.page

The only Nit I find in Machine Elf’s well written post is that Diesel fuel actually burns very quickly…once it gets going. There is an ignition delay that causes trouble, and the injection rate is controlled (slowed) to limit peak pressures, but the fuel itself burns very quickly, which is why it can’t be used in a spark ignition engine. Though this isn’t the reason, it is still a fact that Diesel engines are well suited to low speed, high torque operation.

It is worth noting that High-RPM low torque allows more power for less volume and weight. This is why motorcycles and race cars tend toward those choices. It also tends to lower reliability, which is why airplanes generally don’t. (Porsche’s brief foray into aviation engines being the exception)

This is because the high piston accelerations noted by Machine Elf result in larger forces that the reciprocating parts have to withstand. Main bearings, connecting rod bearings, etc. all wear faster under higher loadings, and any unbalanced loads add to this, especially at high RPM.

If it were simply a matter of ignition delay, then one could just advance the injection timing to enable high-RPM operation. Optimizing diesel engine performance (especially when emissions are being considered) is extremely complex, and is affected by a whole bunch of factors: injection timing, injection pressure, injector hole configuration (# of holes, hole diameter), surface finish inside the injector holes, angle of the injector holes, geometry of the bowl in the piston crown, single injection event versus pilot+main injection versus pilot+main+post injection versus a continuously rate-shaped injection, and probably some other stuff I’ve forgotten about.

I’ve probably oversimplified by saying that Diesel combustion does not happen as fast as the combustion in a gasoline spark-ignited (SI) engine. In a broad sense though, that’s recognized as being true. In fact, the defining feature of the idealized Diesel thermodynamic cycle is constant-pressure heat addition - in other words, heat is released over a finite time period early in the expansion stroke. Contrast this with Otto cycle, which is commonly used to represent the behavior of a gasoline SI engine: the defining feature here is constant-volume heat addition, i.e. all of the heat is released at TDC.

Having said all that, it’s probably more correct to say that Diesel engine combustion just isn’t as strongly influenced by engine RPM as gasoline SI engine combustion is. In an SI engine, turbulence instigated by the intake and compression events stretches and distorts the flame front, resulting in greater surface area, a higher overall reaction rate, and faster combustion. The higher the RPM, the more violent the turbulence, and the faster the combustion - which is convenient, since higher RPM means we have less time available to complete combustion. Yes, the spark timing gets advanced at higher RPM, but not all that much, considering you can make a Formula One engine run anywhere from a few thousand RPM all the way up to 19,000 RPM. If that turbulence weren’t helping to speed up combustion, spark advance alone would not enable such a wide speed range.

Now look at what happens in a Diesel engine. The injector squirts in fuel, which atomizes into tiny droplets. Each droplet is flying across the combustion chamber, quickly evaporating, leaving a trail of fuel vapor behind it. That fuel vapor simultaneously mixes with the air and heats up, and at some point it’s hot enough and mixed enough to ignite. Each individual droplet and its vapor wake are tiny though, and so the size scale of the turbulence doesn’t help mix the fuel vapor and the air as much as we’d really like; we’re very dependent on diffusion, which is a chemical process happening independent of turbulence. Crank up the RPM, and turbulence helps speed up the vaporization/mixing process a little bit, but we’re still mostly relying on diffusion, which doesn’t happen any faster at 3500 RPM than it did at 1000 RPM - even though we need the combustion to happen faster at that RPM.

It is? I must have been absent that day.

Rick, you’re not common.

You are probably right about that. :smiley:

Near as I can tell, it IS common knowledge in automotive circles that different cylinder counts and layouts necessarily have different curves relating their speed and torque and power; also I’m pretty sure it’s incorrect.

I would love to be able to bet people that going into a car dealership or busy repair service waiting room and wondering out loud what layout engine has the “best” curve will get a number of earnest takers. We would see this knowledge is quite common here.

And, actually, it might be true in a dumb sort of way. Nobody is going to put one of the more disdained cylinder layouts into a “muscle car” even if it would work fine, for aesthetic reasons influencing sales. There may well be some correlation between layouts and curves for this goofy reason.

Somebody some day is going to make a mint by selling a noisy, smoky, bad-ass car sporting an engine with one really big cylinder. When it accelerates hard, this thing is going to give bystanders the runs.

Kind of a scaled up lawnmower? Can you imagine the vibration?

I’ve read that longer crankshafts running at high RPMs have to be tougher and need more main bearings (support). Therefore, most inline sixes over the years have been built for good low RPM torque. V-8, flat six (Porsche, for example), and four cylinder engines have shorter crankshafts and can more easily be built to rev higher without crankshaft bending problems.

Reading auto magazines, I frequently encounter statements like “Luxury car owners demand the effortless low-end torque of a V8”, and “Inline-six engines love to rev but suffer from a lack of low-end torque, which BMW has remedied with the turbochargers in the new N54”. I started this thread precisely because I was unable to explain it.

Thank you all for the very informative responses, especially Machine Elf.

It seems to me that certain engine designs, by virtue of their most common applications (e.g. big V8s in trucks and luxury cars, V6s in premium economy cars, I6s in BMW, etc.), have gained a reputation as having certain typical performance characteristics, and sloppy or uninformed journalists associate these characteristics with the engine design itself.

It is very surprising to me that the intake configuration affects the torque/power curves so much. It makes perfect sense, especially because I keep reading about the fancy intake systems used on high end cars, but I am amazed that it is the dominant factor.

Does that mean that, if I had a V6 engine and a V8 that consisted of another two cylinders but otherwise identical (say the V6 is a 3.0L, making the V8 a 4.0L), that they would both exhibit roughly the same torque/power curve shapes?

How much leeway does the designer have to extend the “lever arm” of the crankshaft (i.e. the distance from the axis of rotation to the piston rod attachment point - no idea if that’s the correct term). I would expect this parameter would affect the engine’s torque production, is that correct?

Vastly oversimplified and subject to correction.
The amount of torque an engine produces is a function of piston square inches. A 2L 4 cylinder engine will produce roughly twice the torque of a 1L 4 cylinder engine, all other factors being equal.
Where the torque comes in is a function of camshaft design, ie lift and overlap, plus intake design. A number of cars have variable flow intake manifolds.
How high the engine will rev is a function of stroke length. Longer stroke is lower revving.

There’s that, but another factor is crankshaft torsion. especially problematic if the ignition timing signal is taken off of one end of the crankshaft.

It’s not just the crankshaft, the whole engine is going to have the same problem due to length. That plus the need for 7 main bearings basically means that for the same number of cylinders and displacement an I6 engine is going to be heavier and more massive than a V shaped engine.

The I6 is really an obsolete engine configuration for any modern gasoline car. There’s a reason virtually no one uses it today.

Regardless of RPM, a straight-six or straight-eight engine will need more main bearings. On an inline engine, each con-rod gets its own crank pin; standard practice is to put one main bearing between each crank pin, so e.g. a straight-six engine will have 7 main bearings (5 intermediates + one on each end).

On a V-engine, typically the con-rods from two pistons on opposite banks are affixed to one crank pin - so a V-6 engine will have only 3 crankpins and 4 main bearings, and a V-8 will have 4 crankpins and 5 main bearings.

On a flat engine (A.K.A. a “boxer”), adjacent pistons on opposite banks must reach TDC at the same time in order to be mechanically balanced, so they have to be on separate crankpins. However, they also have to be as close as possible to being on-axis with each other (also for mechanical balance); this means that a boxer twin (found on BMW motorcycles) has 2 crankpins and just 2 main bearings, and a flat-six (found on Porsches , Subarus, and Honda Goldwings) has 6 crankpins and just 4 main bearings.

In an inline six, not only is there perfect mechanical balance, but the combustion events can be spaced evenly across the entire cycle. The result is that they’re very smooth, even at very low RPM, where roughness would be apparent in a V-6, which has imperfect balance (unless a balance shaft is used) and uneven spacing between combustion events. Low-end torque isn’t an inherent feature of the inline six - it’s something that’s designed into the intake system to take advantage of the fact that the rest of the engine is mechanically suited to high-torque operation at low RPM. The inline six-cylinder engine is the overwhelmingly popular choice for larger diesel engines used in tractor-trailers, buses, and other applications. In recent years some pickup trucks have become available with a V-6 diesel option, but I suspect this may have at least as much to do with marketing (“hey, my pickup sounds like a big-rig!” :D).

Pretty much. Assuming the intake runners for each cylinder are ducted back to a common airbox, then each cylinder doesn’t “know” the others are there.

The conrod attaches to the crankpin, which is offset from the crankshaft’s main axis by some radius R. The piston’s stroke length is 2*R. If for example you increase that offset R by 10%, you increase your stroke by 10%. Your piston speeds will increase for any given RPM, and so your redline will decrease. You would expect to increase torque output by 10% - but then, this is because you’ve increased engine displacement by 10%.