Why less cylinders?
 

So it seems the general consensus is to use an engine with less cylinders. This makes sense as using less cylinders means using less gas. My question is why not use more cylinders that are smaller. So for example,

2 Engines, each 1.8 liters:
- Engine 1: 4 cylinder engine (.45 liters per cylinder) (ex. 79x91.5 borexstroke)
- Engine 2: 8 cylinder engine (.225 liters per cylinder) (ex. 39.5x45.75 borexstroke)

In the end, you still have the same size engine, just the bore and stroke is exactly 1/2 the size of Engine 1 with half the cylinders, but double the size cylinder. I'm thinking this might be more efficient as more cylinders will fire per revolution of the crankshaft.

Any thoughts?

More surface area means more heat loss (bigger clynders have less surface area for a given volume)
A V8 is going to have a lot more mechanical complexity too.

Engine 1 has 8 times the displacement of engine 2. In order to reduce displacement 50 % you need just to reduce displacement 50 %. Another option is to divide bore with 2^0,5
(79*2^0,5=59)

Fewer cylinders will give you:

• Less surface area for heat loss
• Fewer moving parts means less mechanical friction (pistons, con rods, valves...)
• Smaller engine (one cylinder with 79 mm bore takes much less space than two cylinders with 59mm bore)

On the other hand, fewer cylinders mean:

• More vibration
• Uneven torque output (bigger flywheel)
• Higher piston speed and/or mass (might be a problem at high revs)

There is also "optimal" piston/cylinder size so its not wise to build small piston V8 engines because you cannot get so good efficiency small v8 as using with correct size pistons.

Also as people already mentioned more parts mean heavier engine and also more expensive engine. OEM try to cut the manufacturings costs anyway they can. Its cheaper to produce big 4 + cylinder engine without turbo than 3 banger with one (two or even three) turbo (s).

However that 3 cylinder is 20+30% lighter and gives better efficiency so fuel consumption is the last choosing point to downsizing from V8 to smaller engines.

Also that 3 banger breaks down faster so its easier to sell new car or new engine after the warranty is run out when that engine fails. that is not fun for the customer but good business for OEMs if everyone is doing it.

a 2L 4 cylinder that's setup similar(target powerband) to a 2L 6 or 8 cylinder engine will be MUCH cheaper to manufacture, lighter, smaller.... all of which OEMs love to utilize.

if you go to some non-mainstream manufacturer, you might see oddballs in either direction(big 4s, tiny 8s), but for a large volume company, oddballs are too expensive.

Engine longevity depends more on how robust the construction is rather than the number of cylinders.

There have been some strange engines over the years... Early Ferraris used 1.6 and 2.0 V12s... And there are big industrial diesels with three, two and even single cylinder layouts.

For modern engines, not only do fewer cylinders mean less friction, they also allow you to more easily package multiple valves, dual spark plugs and/or direct injection hardware. Going down to three cylinders makes it more feasible to make small diesels with direct injection. Vibration may cause longevity issues, or they may not. Four bangers aren't well balanced either, without balancer shafts or DMFs, but they tend to last a long time, anyway.

I think it's because of manufacturing costs.
Usually people interested in mpg also have a limited budget to purchase the car.
It would be interesting to discuss if a shorter stroke would mean less torque?

I have an old SAE paper on exactly this: Bishop, I., "Effect of Design Variables on Friction and Economy," SAE Technical Paper 640807, 1964, doi:10.4271/640807.

For a given power at a given RPM, fewer cylinders have less friction and burn less fuel. But more cylinders with less displacement per cylinder can run higher RPM, and thus get more total power.

When the priority is MPG, the solution is the least number of cylinders that meet overall requirements of smoothness, noise, driveability, etc.

A shorter stroke generally will result in less torque because you lose leverage on the crankshaft. However, increasing the bore will allow for more pressure from combustion on the piston.

My 3.2 v6 is an 81mm stroke by 92mm bore, peak tq of 220@4400rpm and peak hp of 220@6600rpm. Decent low torque, but is pretty motivating above 4k.

(sigh)

will the bore vs stroke "debate" ever end?

torque is largely a function of displacement(regardless of bore/stroke) and compression(not just static compression, dynamic is the key here).

horsepower is calculated from torque at RPM, nothing more.



with VERY few exceptions(all of which are still labratory engines, IIRC) bore/stroke doesn't change(though # of effective cylinders can in certain engines), but you can certainly alter dynamic compression(and some engines even change static compression, but i don't think any are very common).

nearly everything these days uses VVT, which is a good example of that. throttling also plays in as well.



anyways, point is, if you want more torque, you need more dynamic compression or displacement. if you're looking for more power, you need to be able to stuff large amounts of air(and fuel) into the cylinders at higher RPMs, since the horsepower equation "favors" higher engine speeds(since producing the same amount of torque at 2000 RPM compared to 4000 RPM is a doubling in horsepower).

so, in the end, compression and displacement determine torque, the ability to keep large amounts of air flowing at higher RPMs determines HP. the fact that engines that have more bore than stroke GENERALLY do better at higher RPM is due to it being easier to keep airflow up with larger ports that sacrifice low RPM volumetric efficiency(which plays a big part in dynamic compression). there are methods to keep airflow coming at higher RPM without sacrificing low-RPM operation, but they're less "simple". they are becoming more mainstream though(VVT, turbochargers, multiple intake paths, etc).

A bigger # of cylinders have been turned into a premium feature, more than anything. Spending more money for something that would work less efficiently sounds dumb, but many folks are glad to do it...

Marketing. No matter the displacement, no normal, unintelligent member of the human masses is going to pay for a 2l v8 in their compact car.

And all the technical arguments above: Thermal efficiency, Friction, etc.

Torque=force×distance. If you shorten the stroke and leave force unchanged, you get less torque.

you also have less displacement. make up the difference in displacement via a larger bore and torque will be quite similar to what it was before changes. possibly better, since the valves won't be as shrouded.

then there's the whole "a shorter stroke will allow longer rods" argument that comes up in certain circles.

i only know of one instance where a 6 cylinder vs. 4 cylinder is actually benifical.

any 4 cylinder over 2l creates a secondary harmonic, that can distroy engines; harmonic balancers are needed along with heavier crankshafts.

where as the 6 cylinder needs none of that.

gm engines
2.5l I4 110hp; 2.5l v6 145hp; 2.5l I6 154hp

195ci pontiac I4 vs buick 3.0l v6

Can you explain further the secondary harmonic issue?

Also, on the GM engines, how does the 2.5l V6 and I6 get better HP? That is what I was trying to get at with the original question.

In 4 cylinder engines of over 2.4 liters the force of the first and last pistons combustion can develop a rocking motion in the engine from front to rear. This is due to the portion of the energy that actually moves the engine block in the opposite direction of the pistons.

I think it was Mitsubishi who developed the countershafts, typically spinning at twice engine speed that counteract that force.

Basically the reason fewer pistons is more efficient, is the physics of the area of a circle.
Pie R squared. The greater the radius the exponentially greater the area.

regards
Mech

It's not so much accurate to say a 6cyl would be better just based on horsepower. Look at the 2.4L LE5 engine with its 169hp...

I've heard of tractor engines with one huge cylinder that fires once every four or five seconds, and I bet it produces power rather efficiently, considering the low number of explosions per minute and the very small number of moving parts.

That also leads to another point: the selection of a more efficient gear ratio to deal with the torque pulses :D

Not trying to argue here, in fact I agree. By increasing the bore you are increasing the force on the crank. Chrysler gained 30tq/hp by increasing the bore of a 3.2 by 4mm making the 3.5 stroke remained the same but compression increased because of the larger bore.

well, in any case, there's no magic involved, and like every engine decision ever, compromises must be made.

i actually like the thought of a 2 liter V8... or maybe L8... 250cc cylinders have been done FOREVER, i would probably gladly take the small efficiency hit to have a smoother, better sounding engine.... but i'm also an enthusiast of that sort. most of the general population is not.

If sound is a factor, we will prever 3 cylinder engines over 4 bangers. Many motorcyclists seem to enjoy the noise of two cylinder engines...

2.5l I4 ohv tbi
wet manifold tuned for a set rpm range
shrouded valves in cylinder head
counter balancing shafts

2.5l 60v6 ohv mpi
mpi intake(can be tuned better then a wet intake manifold)
cylinder head valves are less shrouded due to mounting
No balancing shafts
possibly better camshaft specs

2.5l I6 dohc mpi
due too the change in camshaft layout, valves have to be canted at an angle in order to be efficiently activated; resulting in less valve shroud
longer intake manifold.
no balancing shafts


ps. there are alot of things that can change an engines efficiency
AutoZine Technical School

Getting back to the original question. For high volume production models, its all about cost. Chrysler had to add a second sets of plugs in the hemi just to meet emissions, but they're cheap as hell to build.

as stated before, less cyls=

lower manufacturing cost

Less energy Loss (friction and surface area for head dissipation)

Better torque for the same capacity (e.g my dad's 2.0 4pot Avensis has more torque than my 6pot 320i)

better/easier packaging (less engine bay space required)



however, gimme a small capacity v6/straight 6 or v8 over a 4pot any day- the noise is just sublime!

building a car as a saleable tool= fewest cyls possible to meet design critera

building it as an engaging, dynamic and emotive "desirable" object, then more cyls and better sounds please!

Have you ever heard of the Hartley V8s? They are 2.8L V8s that redline at 10,000 rpm, make around 400 hp and only weigh like 200 pounds. It's basically two Hyabusa V4 motorcycle engines stuck together. They're pretty nutty... and crazy expensive.

But if that's not quite crazy enough, they also have bored out versions 2.9L and 3.0L that make 450hp, and 500hp respectfully, and if you really want to go nuts they even make twin turbocharged versions that make up to 1000hp. Heaven forbid I ever win the lottery because I would buy a stack of them. :D

Small V12 engines are for one thing. Smooth operation. There are several videos around with the coin test. The operator balances a coin on it's end on top of the engine then starts it up. The coin stays put. THe engine idles and revs very smooth. The power is there at almost any RPM. However they aren't economical. There are a lot of moving parts and friction points so power is lost just moving all of those parts.

Many people buy a V6 because they are smoother and offer more power yet still get decent fuel economy. In production cars in order to get V6 power from a 4 cylinder and still meet emissions you have to use premium fuel and a turbo. Generally a V6 in that case would be more economical due to the lower cost of maintenance and the fact you can use 87 octane.

People don't buy V8 engine vehicles because they are efficient. They buy them for fun or out of necessity. The new trend isn't I4 replacing V6 it is V6 replacing the V8. Case in point is the ecoboost engine. You can get crap loads of HP and torque from that turbo V6. Enough to tow with...yet the vehicle is economical enough to make it your daily driver.

I know everyone will have already addressed the issues of thermal efficiency, friction etc. But there's a really simple one:

Cost. More cylinders means more tooling, more weight, more fasteners, more work to make the engine. If you're building an economy car, there's already next to no profit margin in it. If you can screw together the engine with fewer parts, less tooling, fewer workers, do that.

Actually if they can shoehorn in an existing engine and transmission assembly available from another platform it will save them more money than trying to tool up for a basic engine build. Even if it costs a little bit more per unit to physically build it will cost a lot less if there is a larger run (quantity of engines built in a given period of time). Simply put in manufacturing more units = less cost per unit. Which is why you often find the same basic engine in several different vehicles. Sure marketing, decals and engine covers will vary but the heart is the same.

Nowadays, due to modular designs, many smaller-displacement engines with a lower amount of cylinders can share some internal components with other ones, just like the Vortec 2900 and the Vortec 3700, among others...

Good luck with that.

Q: Can I buy a H1 or H2 engine?
A: Probably not now unless you are an existing customer.

As of March 2012, the H1 and H2 engine technology has been sold. The new owner will further develop the technology for race applications and plans to offer it to other markets in 2013. Hartley Enterprises retains the rights to build these engines in limited numbers for select existing customers and special projects in the US. All projects are taken for 2013.