What’s so Great about the V4 Engine?
The V4 is everything you could want in a motorcycle engine. It has been described as the perfect engine layout, comprising flexibility, smoothness, and purity of output. It is widely used in top level racing, both in superbike and MotoGP, and has had great success. It does have some drawbacks however, which need to be addressed at the design phase.
So ideal is the V4 as an engine, that top level racing has sought to either adopt the configuration (as with Aprilia, Honda, Ducati), or mimic it (Yamaha). The cross-plane crankshaft inline-4 cylinder as developed and produced by Yamaha cleverly mimics the V4 engine’s firing sequence in a more compact inline cylinder layout. So what is it about the V4 that makes it such a special engine, to the degree that other manufacturers would spend millions to emulate it?
Inertial Torque
Inertial torque or more precisely the lack of inertial torque is what makes V engines so special. Present in the standard Inline-4 cylinder engine with traditional flat-plane crank, inertial torque is a by product of the pistons’ movement.
In a normal inline-4, the outer two pistons move up while the inner two cylinder are moving down, and vice-versa (see animation). All 4 pistons are all either accelerating at the same time, decelerating at the same time, or are all stopped at the same time.
Pistons have mass, and therefore momentum. When the cylinders are all stopped, their momentum is zero, this energy is transferred to the crankshaft which speeds up fractionally. When the pistons are all accelerated from rest at the beginning of each stroke, some crankshaft momentum is transferred back to the pistons. This transfer of energy causes a fluctuation of crankshaft speed, a pulsing of torque transfer between the pistons and the crankshaft. To the rider, it represents ‘background noise’ in the engine, distorting an otherwise clear channel of communication between the throttle and the engine, or the rear tyre.
In V engines, when one piston is stationary, the other (as they are linked to the crankshaft at the same point), is at its maximum speed, cancelling out this inertial torque effect. The cross plane crank with 90 degrees between crankpins has the same effect in an inline configuration.
Bikes with traditional inline-4 ‘screamers’ are fitted with cush drive rubbers to prevent these not inconsiderable forces from damaging the drive train at high engine speeds. Cush drives rubbers also have the effect of masking the direct feel between the rider and the tarmac. Imagine two sprinters lining up for a race; one is wearing running spikes, and the other is wearing rubber-soled trainers. Who has a better feel of the track surface below when they unleash all that force from those powerful legs?
If you have ever ridden a V-Twin, a Boxer Twin, a V4, cross plane crankshaft R1, or Triple (all have zero inertial torque), you will notice that while they are not as smooth as a standard inline 4 cylinder, they spin up more freely and purely, and they communicate drive to the rear wheel more clearly, giving the rider more feedback, and more confidence when applying the throttle early in the corner.
If you have a look at top level racing, you will also see that V4 and cross plane crankshaft engines are continually demonstrating their prowess on track (although recently, traction control systems are levelling the field somewhat); Honda V4 RC45, Aprilia RSV-4, Honda RC211-V, Ducati Desmosedici, Yamaha M1, Yamaha YZF-R1.
Uneven Firing Sequence
What is it about the firing sequence of V-Twins, V4’s and Cross-plane crankshaft 4 (CPC4) which seems to give them better rider feel, and better early drive? Inertial torque plays a part as we discussed above, but does the firing sequence also come into play?
The 90 degree V-twin has a firing sequence with a delay of 270 degrees and 450 degrees. As both cylinders are joined to the crankshaft at the same point. The first cylinder fires, and the second fires three quarters of a rotation later, then the first cylinder fires one and a quarter turn later at 450. (In the following examples each entry is 90 degrees of crankshaft turn, 1 is a fire, 0 is no fire)
90 degree V-Twin: (1–0–0–1–0–0–0–0–1–0–0–1–0–0–0–0–1–0–0–1–0–0–0–0-)
The tyre has a break of four large ‘gaps’ in the power between the second and first cylinder firing, in a screamer engine, the firing sequence looks like this, firing every 180 degrees.
Inline 4: (1–0–1–0–1–0–1–0–1–0–1–0–1–0–1–0–1–0–1–0–1–0–1–0-)
The V-4 engine as installed in the Honda VFR800 series fires as follows (180, 90, 180, 270)
V4: ( 1–0–1–1–0–1–0–0–1–0–1–1–0–1–0–0–1–0–1–1–0–1–0–0-)
A tyre is elastic, and the way it grips the road and exerts a force on it, is down to the movement that the tyre makes at the level of contact with the road. The movement is within the tyre’s construction, as well as being a certain amount of slip at the contact with the road surface.
There is a theory that if the tyre is being pounded by even firing sequences, it never gets a ‘breather’ from the pummelling; never gets a chance to relax back to rest position so it can key in to the road surface. Static friction is much greater than dynamic friction, one the traction and the tyre is spinning, it is much easier to keep it spinning. If you have ever been on a ride in low grip conditions you will have appreciated this. You want the tyre to spin back down to normal grip conditions rather than lock into the road suddenly. Chopping the throttle suddenly will give the tyre a chance to grip, and this is where a high side occurs. Electronics ‘think’ much quicker than human reactions and can help in this regard, as can a rider who isn’t freaked out about the rear end sliding about.
V4 and a V-twin firing sequences give the rear tyre a rest (red zero’s in the sequences above) but whether this improves the tyres ability to grip is not easy to prove without getting into the complex world of tyres and how they behave. Theoretically, if someone could build a 4 cylinder engine with a regular firing sequence but also with zero inertial torque, this could be compared directly with the standard flatplane crank I-4.
The 90 degree V-4 is essentially two 90 degree V-twins side by side. Each pair of cylinders shares the same crankpin, and they are 180 degrees (flat plane crank) out of sync.
A flat-plane crank as employed in a 90 degree V-engine (including V8’s that Ferrari and other exotic manufacturers use) has the same zero inertial torque characteristics as a cross-plane crank in an inline 4 engine.
To prove that a V4 is two V-twins, let’s take two traces of the V-Twin above and overlay them offset by 180 degrees (each 1 or 0 represents 90degrees in the sequence):
1–0–0–1–0–0–0–0–1–0–0–1–0–0–0–0-
+1–0–0–1–0–0–0–0–1–0–0–1–0–0-
_______________________________
= 1–0–1–1–0–1–0–0- 1–0–1–1- 0–1–0–0-
This is the V4 VFR800 firing sequence
Out of the two; inertial torque and uneven firing, the most important contribution to improved drive is zero inertial torque.
Smaller Cylinders = Higher Power Output
There is a limit on how light we can make pistons, a limit on the materials we can use (without spending loads of money like in MotoGP), and there is a limit to how over-square an engine can get before it doesn’t produce enough low down torque to be considered flexible enough. Manufacturers are pushing this limit as they improve low down performance using amongst other things: VTec; Variable length inlet trumpets; Exhaust valves; dual injectors; friction reduction; lighter crankshafts.
As I explain in my book, the more cylinders an engine has for a given capacity, the smaller the pistons weigh, and the faster they can be accelerated up and down without flying off the end of the con-rods. As power is torque multiplied by rotational speed, the lighter the piston, the higher you can spin the engine and the more power you can make. Torque is the size of the bang, Power is the rate at which those bangs come (hence related to engine speed).
In boxing, jabbing with bare-fists is easier than jabbing wearing heavy boxing gloves, as that weight needs to be accelerated and decelerated with each stroke, and this places a strain on your arm (or in this analogy, the connecting rod).
Manufacturers can play tricks to reduce internal friction in an engine, and therefore to allow more useful power output; changing lubrication systems, valve gear design, and by flowing the gases easier, but smaller pistons can travel faster and therefore produce more power. Have a look at the new R1 engine mods.
A V-twin is cheaper to produce that a V4, as it has half the number of cylinders. So expect the budget on those two cylinders to be appropriately high to compensate. If you want a V-Twin to produce as much power as a V-4 of the same capacity, you are going to need to spend some serious cash.
In superbike racing, V-Twins are allowed an additional 200cc of capacity to put them in line with the 4 cylinder engines (Ducati complained that they were having to spend excessive amounts to keep their 1000cc twins competitive with the fours, and threatened to quit if they weren’t allowed to up the capacity in line with their road bikes). In the past it was 750cc fours versus 1000cc twins, but nowadays it’s 1000cc fours (including v4) vs. 1200cc twins.
Flexibility
When it comes to internal combustion engines, what we like to see is a flexible power plant with a good spread of power throughout the rev range. While it is nice to have the heady top-end rush of a screaming four cylinder bike, on the road as well as on the track, the more flexible the engine output is, the quicker the rider can get on the throttle and pull himself out of a corner. The engine which makes all of its power at the top of the rev range is going to struggle in the lower speed corners.
V-Twins, for anyone who hasn’t had the pleasure of riding one, are flexible engines. They are to the motorcycle what turbocharged engines are to the motorcar; producing plenty of drive throughout the rev range. This is because big cylinders have big bangs, and the bigger the bang, the bigger the torque. Power is the rate at which those bangs arrive.
To return to our boxing analogy, the heavyweight boxer packs a bigger punch but doesn’t throw as many punches per second (V-twin), whereas the bantamweight boxer throws so many lightening quick jabs you end up losing count, but each has less effect. Each boxer may inflict the same damage, but the bigger boxer packs a bigger punch and may even knock you out.
V-Twins, because they have larger pistons, tend to have more inherent vibration, whereas the smaller pistons of a 4 cylinder cause less vibration. This is somewhat offset by the irregular firing sequence of the V4 which adds its own imbalance, making the engine somewhat ‘lumpy’ below around 3,000 rpm.
The V4 then, has smaller cylinders but each pair is arranged as a V-Twin, which gives a characteristic somewhere between the drive of a V-twin and the outright power of an Inline 4 cylinder.
Flexibility
A V-twin is a narrow engine. Both pistons share the same crank pin, and while there is an offset, the arrangement can produce a very slender motorcycle. An inline three is clearly narrower than an equivalent inline 4. But the V4 (because each pair of pistons is 90 degrees apart), may even be narrower than a three cylinder of equivalent capacity.
So the V4 gives you a narrow engine, but it does unfortunately mean you have issues in fitting the engine in lengthways. They are also expensive to build, and so are only tackled by companies with large bank balances (Honda and Aprilia).
Sportsbikes handle well with a short wheelbase, and large power outputs are tamed by longer swinging arms. The radiators at the front, and the shock positioned at the rear of the engine are often moved to accommodate the lengthy V4 engine, or sometimes the V angle is reduced to make it more compact (Aprilia RSV-4), but it doesn’t make them easy to work on.
Honda’s VFR800 and VTR1000 used side mounted radiators to allow the engine to be moved forward for better weight distribution over the front forks. Equally, Suzuki’s TL1000S adopted a rotary shock without a great deal of success.
The Sound
I like inline-4 engines. They are as smooth as a fine glass of Bordeaux (especially the 600's), and they keep on pulling. But the don’t have that much character. The fruitier engines are pretty exciting in race configuration; 1000cc production bikes like the Suzuki GSX-R1000 and the Honda CBR1000RR Fireblade are pretty exciting machines, that are also surprisingly easy to just get on and go fast. But when you’re looking for something a little different, the V4 or the cross plane crankshaft I4 have a lot of character, and they sound incredible, like a mini V8. You cannot beat the sound of a thrumming V-4, it’s like (in fact identical to) the middle four cylinders of a traditional V8 layout.
The Future
Yamaha has really shown the way with its use of the cross-plane crankshaft in production motorcycle classes, and its success in the 2008/09 season is testament to the solid basis of the format. It has made other manufacturers take notice. Those with less financial clout than Piaggio (Aprilia’s owners) and the big H, can now produce something as effective as a V4 in a more compact inline 4 casing, without the complicated and expensive design and packaging issues already discussed.
Interestingly Suzuki is relaunching its MotoGP campaign for 2015 and is adopting the cross-plane crankshaft design that Yamaha switched to in 2004 when Rossi defected from Honda.
Perhaps we will see Suzuki’s road bikes going the same way as Yamaha’s. Given the fact that all GSX-R engines are broadly the same design, what they use for the ‘daddy’ 1000cc, is also likely to be used for the 600cc and 750cc also. It would make things rather interesting. I for one would be queuing up to test ride a GSX-R750 with a cross plane crankshaft I4. I’m not sure it warrants use on a 600cc. Here’s hoping…..
Originally published at www.shiny-side-up.net on January 9, 2015.
