Some engine configurations are inherently better balanced than others. The two most naturally balanced are inline 6s and V-12s. Most others require balance shafts to smooth out the roughness.
But all engines are compromises based on the application. An I-4 is great for smaller front wheel drive cars, where space is at a premium and you’re driving wheels right below the engine. A V-8 is well suited for a RWD sedan with a fairly long and low hood.
Thanks. I did not realize there really are no V-4 engines and the answer is vibration modes (it seems crazy to me a V-4 would have more vibration than an I-4 but the people building these things know waaaay more than I do about it).
V4: Similar to the v-twins, this configuration uses 4 instead of two cylinders. Virtually all the bikes in the last decade of the two-stroke GP500 era were fours. These engines are compact and have almost zero vibration.
In road cars, commuter cars, don’t worry about cylinder count all else being equal. But as has been stated, more cylinders is generally smoother. But I’ve been in some silky smooth 4 bangers and some harsh v8s.
In racing tho, a v4 was very successful fairly recently. Though it was a hybrid and made a shit ton of power from electric. And the evo variety even beat some F1 lap times briefly at some circuits.
There are two different issues in play here. Balance, and the vibration that lack of balance creates, is one. It’s complicated because there are multiple ways the engine can lack balance. Certainly, having many more cylinders can help with this. But small increases could make it worse.
The other issue is how many ignitions there are per revolution. To the extent that each one is a separate little kick in propulsion, more ignitions per revolution is always smoother. In this case, going from four stroke to two stroke has the same effect as doubling the number of cylinders.
The case that is particularly interesting to me is diesel tractor engines. Over a wide range of horsepowers and brands, three cylinder four stroke inline engines are common. I’ve had two of these. At first it struck me as an odd choice, but it’s common enough there must be a good reason. Anybody know???
I think you have to take into account the primary use of the car. I live in an urban/suburban environment. I commute to work. I’m not going to drive faster than 85 MPH, and those opportunities are very limited. An 8-cyclinder is a gas eating overkill as far as I’m concerned. My 4-cyclinder Honda C-RV gives me about 27 MPG, it is peppy, and it is reasonable in size.
There are tradeoffs. The balance issues WRT I4, V4, and V8 layouts has been discussed, and this is not just about having smaller and more frequent power pulses, it’s about which way the various masses are moving and when. If you run through the math and the kinematics, you find that the piston movement is not just a simple sine wave: the downward acceleration at TDC is more brief and severe than the upward acceleration at BDC. There’s also the lateral movements of the big ends of the connecting rods. Inline 6-cyls and V-12s are about as good as it gets. I4 and smaller engines can get by with good mounting and auxiliary balancing shafts that help counteract some of the inherent imbalance.
Other concerns include efficiency, packaging (i.e. cramming that thing under the hood), and cost.
Cost seems obvious. If you have twice as many cylinders, then you also have twice as many pistons, rings, valves cam lobes, cooling passages, and so on, that need to be manufactured and assembled.
Larger cylinder dimensions mean a lower surface-area-to-volume ratio, which means less of the heat of combustion gets lost to cylinder walls, which tends to improve efficiency. The Honda Fit in Europe comes with a 1.2-liter I4 engine (.3 liters per cyl); and the latest Chevrolet Corvette can be had with a 6.2-liter V8 engine (.775 liters per cyl). If you wanted to improve the SATV ratio on the Fit, you could make the cylinders bigger, but that would increase the overall displacement, which would have you running the engine at a lower average throttle setting, reducing efficiency. Or you could keep the total displacement the same, but switch to a 2-cyl or 3-cyl, but then vibration would be worse.
More cyls of course also means more parts moving against parts, so more friction and worse efficiency.
I think because on a diesel tractor, vibration is a low concern and efficiency is a high concern. Get the cylinder count down and get the SATV ratio down (also, there’s the “fewer parts = less cost” issue). Mitigate the vibration with good engine mounting, and fit a big-ass flywheel to get you through those mondo compression strokes.
In your case, an 8-cyl engine would inevitably have a larger displacement, making it capable of more torque and power; this means you’d be driving it with a light touch on the accelerator pedal. Engines are inefficient at light loads, which is the main reason big engines in small vehicles make for crap fuel economy. For the OP, the thing to compare mentally is the 2.4-liter I4 in your CR-V, and what your CR-V would be like with a 2.4-liter V8 . If the latter engine existed, it would probably be smoother, more expensive, and get poorer fuel economy, but not as bad as, say, a 5.7-liter V8.
The smallest cylinder displacement I’m aware of on a mass-produced vehicle is the BMW K1600 motorcycle, which is fitted with a 1.6-liter transverse I6 engine: just 0.267 liters per cylinder. I’ve ridden this bike before, and it’s smooth and powerful. No idea about the fuel economy, but then nobody buying a motorcycle in that price range gives a rip about fuel economy.
3 cylinder engines, and multiples of 3 cylinders have exceptional primary and secondary balance. From Wiki on straight 3 engines: “A crankshaft angle of 120 degrees is typically used by straight-three engines, since this results in an evenly spaced firing interval. Another benefit of this configuration is perfect primary balance and secondary balance, however an end-to-end rocking couple is induced because there is no symmetry in the piston velocities about the middle piston. A balance shaft is sometimes used to reduce the vibrations caused by the rocking couple.”
Note the picture of the 3 cylinder diesel at the top of the article. I believe for lower RPM diesel engines the balance of the 3 cylinder engine is very effective in reducing vibration.
Enzo Ferrari, famous for his 12-cylinder engines, also used 6 and 4 cylinder variants in his cars and he experimented with a 2 cylinder variant for even better volumetric efficiency. It ran so rough it broke its crankshaft, dynamometer, and/or testing bench (stories vary), so he stuck with at least four cylinders.
Thanks. My hunch was that three cylinders were just a cheap step down from inline-four and -six cylinder engines, which could all be made on the same equipment with many of the same parts. I figured smoother running tractors just weren’t a high priority for famers getting the cheapest option. Turns out there’s a lot more to it than that. Ford used some three cylinder engines in the recent economy cars (mentioned in the same wiki) and they really weren’t that bad. Now I know why.
I think the fun one is a Volvo 5 cylinder engine. I think they use large counterweights somehow to balance the engine. Modern engines are much less affected by vibration due to fuel injection and electronic ignition so every combustion in each cylinder is virtually identical, but they’re still reciprocating engines and will have to deal with that.
The vibration modes are well known and can be calculated. Inline 4 cylinder engines have their major vibration node at about the top of the piston travel a specific distance off to the side. Intensity varies with bore, stroke, connecting rod angle, etc. The Lotus 4 banger from the 1960s made terrific power but had a vibration so strong at the node it could cause fuel in the carb to froth and lean the engine out. Colin Chapman in his simplistic approach simply hung the 2 twin choke carbs on resilient mounts. They would flop all over the place.
60 degree V6s are fairly well balanced but not like an inline six. GM chose to build their V6 on the tooling for the V8 so they were 90 degree engines. Damn they could be harsh.
From what I understand, for a given displacement, an engine with fewer cylinders will produce more torque at a lower RPM, while the engine with more cylinders will rev higher. For example a 5 liter V6 would be more suitable as a truck engine (back in the 1950s and 60s GMC made a large V6 like that) – slow, but good for pulling heavy loads at low speeds. A 5 liter V8 would be more suitable as a sports car engine (like the one in the Mustang). Although I don’t really understand all the reasons why that is the case.
ETA: I missed the discussion of 3 cylinder tractor engines earlier, but I suspect this might be the reason. And scaling up, as I understand most semis use something on the order of an 11 liter 6 cylinder diesel.