What is the advantage of a V (or horizontally-opposed) engine over a straight engine?

There are a multitude of considerations but, external packaging is a primary concern in the automotive world. There is some concern over engines with longer crankshafts particularly in higher speed engines because of the tendency to whip a little at higher speeds if they are not stiff enough. They can be stiffened but, this usually results in larger bearing journals with accompanyong higher bearing speeds which are not desireable. There is also the problem of the harmonics of torsional vibration with longer cranks. Many of these design problems are addressed using the shorter cranks of a "V" config. As fewer cylinders are used, many of these problems are greatly diminished as in the case of the inline 4 cylinder which has relatively low frequency power impulses compared to a 6 or more cylinder engine. Many of these problems re-surface at higher engine speeds. Most of these problems are addressed by design but, external package config is the major concern in the automotive world.

As far as timing, etc, yes, engine design can be greatly influenced with timings and the basic engine config can greatly influence the power and performance characteristics. This can be particularly noticeable between engines like an inline 6 and a V8 or V12 of the same displacement. Even though the V's would generally have higher maximum horsepower output than the 6, the 6 would generally pull harder at lower engine speeds. This concept is extensively used in heavy duty trucks where inline 6 cylinder configs are the rule rather than the exception because of the higher low-speed torque production which can make the vehicle much easier to drive.

The above is a very simplistic answer on a subject on which entire libraries have been written by people much smarter than I. I only hope I have offered some insite.

Lugnut

I suppose also that it's easier to cool a V engine with two separate n-cylinder blocks than an in-line engine with a single 2n block.

Oh, so for the same capacity and number of cylinders, does the V engine develop more power but less low-end torque than an in-line engine? Didn't know that.

Does a V engine run more or less smoothly than an in-line engine, given that in an inline engine the explosions occur at equally-spaced intervals whereas with a V engine the two banks of cylinders are offset by maybe 45 degrees.

In an in-line engine with more than 4 cylinders, is it normal for *all* the cylinders to be in-phase (ie all at TDC or TDC+180 degrees) or is there an offset? For example, with an 8-cylinder engine, do two cylinders start on a power-stroke at exactly the same time, two on induction etc, or is one group of four cylinders offset from the other by 90 degrees so its power strokes will occur in-between those of the other group of cylinders, as if it were a

90-degree V engine?

Cooling is a design problem. There is generally not a significan difference in a properly designed system. It is normal, however, for the cylinders at the rear of an engine to run a bit warmer than the cylinders closer to the cool water at the pump.

Not necessarily. It depends on which component of "power" you are talking about. They have different useable characteristics. An engine with lots of torque at lower speeds will be easier to drive in everyday driving but, the engine that produces more horsepower at higher speeds may have better maximum acceleration characteristics. A small truck with only a 100 HP inline 6 cylinder engine may be able to drag a 400 HP honda Civic around backwards all day because of it's torque advantage at low speeds while the Civic can run off and hide on acceleration because it's horsepower advantage. The nature of the work to be done should be the guide when selecting an engine for a specific job.

No. They both can be designed to be very smoothly. Just because the cylinders appear to be offset has nothing to do with the timing of the power impulses.

No. The crankshaft is made so the the cylinders are offset from one another resulting in even power impulses. You may want to pick up a book on basic engines at the library to see this.

Start here for very basic intro:

Lugnut

I am going strictly from memory, and there may be those who find fault with my perception of the subject but, as I remember the practice versus theory, all engines do not have even power pulses.

In particular, some of the early Buick V6 engines did not. It required a special crankshaft design in later versions to get the even fire operation. The oddfire versions were tamed with motor mounts,and other techniques, and gave surprisingly good durability.

The ability to balance an engine varies a lot with the engine design. Some configurations cannot be balanced perfectly, while others can.

Thanks, Lugnut, for you comments on engine design.

Have you sized an inline 8 or 12 lately? Unless you want a hood that is twelve feet long, it makes sense to put them into a V configuration. There other option is a horizontally opposed config, but it is wider than a V.

Martin Underwood wrote:

Most engines are designed to fire at evenly-spaced intervals. A four will fire every 180 degrees of crankshaft rotation, a six every

120 degrees, an eight every 90 degrees, a 12 every 60 degrees. Whether it's an inline or vee or opposed makes no difference.The engine is designed internally to get the even firing. The Harley-Davidson engine is one example of an engine with uneven firing, giving it the distinctive lope. Inline sixes can pull better at low speeds because their redlines are often lower due to the longer, more flexible crank and larger bearing diameters, so the designers get the required power by making them less oversquare ("square" meaning that the cylinder bore diameter and stroke are the same measurement, and "oversquare" meaning that the diameter is larger than the stroke). An engine having a relatively longer stroke will generate more torque and lower RPM, just as a bicycle with longer pedal cranks will climb better. I once owned an Auster aircaft. This airplane had a deHavilland Gipsy Major engine in it of around 6 litres displacement (350 cu. in.), all in four cylinders. The bore was 4 5/8" and the stroke 6 1/2", making it way undersquare. It delivered 145 hp at 2600 RPM, giving it a torque rating at that RPM of 293 ft-lb, rather awesome for an engine of that displacement. Horizontally-opposed engines are naturally balanced, eliminating the need for massive counterweights and making the engine lighter, though some larger opposed aircraft engines do have counterweights that are loosely mounted so that they can move slightly on the crank to reduce some forms of harmonic vibration. An opposed engine has a short, stiff crank to make things safer. An opposed engine has a low profile, making it ideal for the front of an airplane or the engine compartment of a low car. Opposed engines in aircraft are commonly found in 4, 6 and 8-cylinder versions, and Lycoming once built a 12-cylinder engine that was never produced. Most of them are air-cooled. A liquid-cooled opposed engine has no need of cooling fins so the cylinders can be placed closer together, shortening the crank further. Liquid-cooled V-12s were used in many bombers and fighter aircraft of WWII. The best was the Rolls-Royce Merlin, a 1647 cu-in. engine that produced more than 1500 hp, and a version of that engine was run for 15 minutes while producing 2640 hp. The V-12 block was mostly aluminum with steel cylinder sleeves, and was light and fairly narrow to fit in tight cowlings. It was geared so that the engine ran at 3000 RPM and the prop turned at less than 2000. (Big propellers can't be run at high RPM. The tips must be kept below supersonic speeds, and centrifugal forces get too high at higher RPM.)

Dan

I believe the Viper's V10 is another one like that.

Honestly, I'm no mechanical engineer or automotive designer... but I'm pretty sure that 90% of the layout of a modern automotive engine comes down to marketing and packaging. (and by modern, I mean since about 1960.)

A V8 is more "impressive" than a 6, which is "better than" a 4. Inline 6's were "old", so a V6 was newer and therefore "better."

American musclecars have always (Buick 3.8 turbo excepted) had V8's, so even if you could make 400hp from a V6, it probably wouldn't sell as good as the V8 version.

Once you've invested a lot of engineering $ and tooling $ into a certain design, you're generally not inclined to toss it.

They've tried a couple of other designs too, like VW's VR6 and W8...

They all try to claim each design is better... I don't buy it. I have a Subaru with a flat-4 - according to them it should have a lower CG than an traditional inline, but I'm not sure if I buy that, because once installed in the car, it's not THAT much lower and definitely a LOT wider. (I bought the car for awd, not because of the engine, and it's not a sports car, so a 1/2" reduction in the CG isn't going to make a huge difference.)

Ray

You are correct. My response is intended to be very general for the OP who appears to have extremely limited knowledge od how the engine works. There are exceptions to almost every rule and the Buick engine was one of them. There are other oddballs out there that did not work nearly as well but, the Buick had some natural feature to overcome. You may also want to look at the Radial 9 aircraft engines for some interesting features.

Lugnut

you all should just be cool and get rid of piston moters because it is all about the rotory moter!

>

The rotaries I've seen are niether fuel efficient, nor, big on low end torque when compared to piston engines outside a relatively narrow operating range. This may cease to be as much of a problem as transmission development advances that will provide for operation in that relatively narrow range. Rotaries, in general, are still in need of lots of development to compete with more traditional offerings in the mass markets. The piston engines will go only when/if anyone ever comes up with a viable replacement. That is still a tall order even as outdated as some think the piston engine may be.

Lugnut

Horizontally opposed six and twelve cylinder engines have a high degree of balance. Same for inline six and twelves.

The same is not true for horizontally opposed twos, fours, eights, etc.

I've just skimmed the responses, so if this has been stated before I apologize.

The V engine (and by extension, the H engine) are significantly shorter in 6-cylinder and up applications. this is a big bous for packaging.

There are some engines that have good inherent balance. An I-6 or V-12 has almost perfect natural balance. A V-8 is pretty good. A V-6 is awful. So an answer as to one layout is better than any other depends on how many cylinders we're talking about.

The one thing that I haven't seen mentioned is center of mass. The crankshaft will almost always end up in the same place in the chassis, so the horizontally opposed engine will provide the lowest center of mass when installed; the V-engine next, and the inline worst (unless it is slanted, as a certain American manufacturer famously did a couple decades back.)

nate

Better qualify that. The textbooks disagree with you.

Dan

A shorter crankshaft is more rigid, less prone to torsional vibrations, and generally stronger. Inline sixes are about the longest practical crankshaft length for car-sized engines (IOW, you can go a lot longer in a locomotive engine where you don't CARE that the crankshaft weighs 4000 pounds). Inline 8 engines and V-16s were obviously used back in "the day," but even then they were running up against the limits of the crankshaft as horsepower levels increased.

Most V-type engines are actually a little less inherently well-balanced than their inline counterparts. A notable exception is the V12, which is just two inline sixes and therefore just as smooth as an inline six- commonly considered the most "naturally balanced" engine configuration of all. A typical V8 is NOT timed like two inline 4s, and for good reason- inline 4s SUCK from a vibrational viewpoint. That's why so many inline 4s are hobbled with balance shafts- to cancel out the remaining undesirable flex and vibration modes caused by having the inner two pistons going up and the outer two going down in unison. The so-called "90-degree crank" v8 used in virtually all passenger car v8 designs is much smoother than a 4-cylinder, though not as nice as a straight six. There are "180 degree" or "flat-crank" v8s that are timed like two siamesed 4-bangers, but they are not used in production cars because of the vibration problems. They are used in some racing series (IRL used them for a while) because of desirable breathing characteristics and manifold designs allowed by the timing, but they are just too rough for daily use.

True, but I look with such derision on those engines that I hardly call them "engines." :-/ Odd-firing is a half-assed cheap way out of building a more complicated crank. But it does work, or Harley-Davidsons wouldn't run at all :-p

True of any 90-degree v6.

The oddfire

Because the crankshaft was WAY overbuilt for the low power they put out, yes. It would never have survived the power levels that it got to later in even-firing form in the turbocharged Buick GNX and the later supercharged FWD Buicks and Pontiacs of the 90s.

As long as you can make an engine EVEN FIRING (the discussion above) you can take care of the rest of its balance idiosyncracies through crankshaft counterweights or additional balance shafts driven by the timing chain or belt. The proof of that is that one of the inherently WORST engine layouts in terms of inherent balance is the inline

4-cylinder- but look at how many gadzillions of them have been built and continue to be built. Its arguably the most common engine layuout for cars. Also look at the popularity of 90-degree v6 engines in even-firing form. They have a lot of inherent imbalance, but its all 2nd and 3rd order stuff so long as the are even-firing.

The Viper and Dodge Ram truck V10 are a very special case. Unlike the early Buick V6 or the Harley-Davidson twin that are truly "odd firing" in that there are gaping holes in their firing pattern, the V10 has pairs of cylinder that fire "close" and pairs that fire "farther apart," but a "close" pair is ALWAYS follwed by a "far apart" pair and vice-versa so that overall, the engine runs perfectly smoothly. But it has an odd double-toned exhaust note as a result.

And I believe the reason a Viper has a V10 instead of a V8 is because of Moore's Law. If some is good, Moore is better. The current Dodge Charger SRT8's 6.1L V8 Hemi makes more power than the original 8L V10 Viper engine. But a V10 is more cylinders, therefore it's better.

And all the car companies are guilty of that kind of stuff - how well would a V6 Corvette sell?

Ray

A) The 6.1 Hemi didn't exist when the Viper came out. It wasn't even on the drawing board. The biggest engine Chrysler made at the time was the

360 (5.9L) v8 at only 230 horsepower and tuned for maximum low-RPM grunt because it was only used in trucks. The v10 is loosely based on the 5.9 architecture with two extra cylinders.

B) The V10 is going away in a couple of years, in favor of (you guessed it) the Hemi.

There is a downside, at least for the hardcore Viper enthusiasts. The V10 is EXTREMELY under-stressed in factory trim. The thing is just loafing when its putting out 500 rear-wheel horsepower, and many aftermarket tricks (both with and without forced induction) easily put it in the 1000 rear-wheel horsepower range. The 6.1 Hemi is a GREAT engine, but whether it can match that potential is very questionable.

Is it much of an advantage for a four-cylinder engine, which is what my original question was about? I appreciate that for a 6, 8 or 12 cylinder engine, you'd need a very long engine compartment (or a very wide one if the engine was transverse), and there may also be crankshaft vibration problems due to the greater length.

But is there any advanatge of V4 or H4 over straight 4? If not, why did they go through a phase of trying it in the late 60s, I wonder?

People have mentioned vibration of certain engines. How about engines with an odd number of cylinders, like the 3-cylinder engine that's fitted in my mother's Daihatsu and some Daewoos, or the 5-cylinder engines that (I think) Audi made in the 1980s. My mum's Daihatsu is surprisingly good, though its automatic transmission has the gearchange thresholds set at very high revs so the engine spends a lot of time at high revs (which makes it noisy) - presumably to get decent power out of a mere 900 cc engine! Maybe I'm biassed because I'm used to my 2-litre diesel which rarely goes about 3000 rpm and accelerates fine in third gear on a roundabout at 1000 rpm.