V8 engine crankshafts are roughly divided into two types, one is a cross crankshaft and the other is a flat crankshaft. The biggest difference is that the angle between every two crankshafts is 90 degrees instead of 180 degrees. The plane crankshaft V8 engine has a simple structure and small inertia, which is conducive to high revolutions and engine response, with super vibration...

 

There are two concepts of engine mechanical vibration: first-order vibration and second-order vibration

 

 

E.g. three-cylinder machine

At any moment when the crankshaft rotates, not only does the number of up and down pistons always vary, the movement directions of the pistons of cylinder 1 and cylinder 3 are always opposite, causing the engine to not only vibrate up and down, It also oscillates back and forth. If you want to use it on a mass-produced car, you must equip it with a balance shaft, otherwise you can use it to drive an electric toy. As the saying goes: the three-cylinder shocked the world.

 

But the common four-cylinder machine

It seems that two cylinders move up while two cylinders move down. Is this a perfect engine?

 

Taking out half of the four-cylinder engine to analyze separately, it is not difficult to find that due to the geometric configuration of the crank connecting rod, the speed of the upward piston is always faster than the speed of the downward piston, causing the engine to vibrate up and down every 180 degrees of the crankshaft. .

 

Solution? A balance shaft that rotates twice as fast as the crankshaft. It can be said that after Mitsubishi first applied the dual balance shaft to the mass-produced 4-segment engine in the 1970s, this type of engine really had a future.

However, the early four-cylinder engine crankshaft did not even have a counterweight. In addition to the machining process problems at that time, the engine speed was lower than that of the current diesel engine.

So in the 1910s, Cadillac and Ford designers wanted to solve the vibration problem through a 90-degree angle and counterweight. (But in theory, the plane axis does not need this design)

The side valve V8 and the simple flat crankshaft at that time

 

The advantage of the 90° included angle engine is that the balance weight on the crankshaft can be used to offset the vibration torque generated by the movement of the piston in another row of cylinders. This principle is applicable to a 90-degree V-type engine with several pairs of cylinders.

For example, when the upper cylinder moves upward, the counterweight moves downward. While rotating counterclockwise, the speed of the counterweight points to the lower right after turning at 6 o'clock, but the piston moving from the right to the left counteracts this moment.

 

But in the 1920s, the engine speed increased, and the problem of secondary vibration became more and more obvious, so most of the mass-produced V8 engines began to be equipped with a cross crankshaft.

The biggest difference between the cross crankshaft (top) and the plane crankshaft (bottom) is that the angle between every two crankshafts is 90 degrees instead of 180 degrees. The plane crankshaft V8 will have the same secondary vibration problem as the straight 4 engine, and the 90-degree interval between the two rows of cylinders will also cause the 180-degree vibration to be superimposed. The cross crankshaft is because the difference between the two sets of crankshafts separated by 180 degrees is 90 degrees instead of 180 degrees. The frequency of the secondary vibration is only half of the plane crankshaft, and the amplitude is greatly reduced.

 

Remember the advantages of the 90-degree engine? The problem is solved after adding the counterweight

But here comes the problem. Since each row of cylinders has two pistons reaching top dead center at 90-degree intervals, no matter how the ignition sequence is arranged, each row of cylinders will have two ignitions at 90-degree intervals, resulting in serious exhaust interference (that is, the general V8 Engines are similar to the cause of exhaust noise from agricultural machinery).

Therefore, in order to increase the scavenging capacity at low speeds, the general civil V8 will design an H-type or X-type balance pipe in the middle of the exhaust, and use the pressure difference between the two exhausts to reduce the influence of exhaust interference.

Some performance-focused V8s use a more tangled design. For example, the exhaust pipe of the Ford GT connects the adjacent ignition cylinder to the exhaust manifold on the other side. What's more (the frenzied BMW) does not hesitate to exhaust the exhaust. Measured to the inside of the V to use more complicated exhaust manifolds

So the cross crankshaft is not a good thing for high-performance engines. Although the vibration is small, the heavy counterweight causes the internal inertia of the engine to be too large, which is not conducive to the sensitive engine response and the realization of high speed, not to mention the weight reduction. In addition, exhaust interference is also a major taboo of performance engines. So the European high-performance V8 engine still insists on using a flat crankshaft.

 

The plane crankshaft V8 basically welds two straight 4s together. Since the pistons running upward and downward are always in pairs, there will be no primary vibration problem, but the double secondary vibration must require a heavier balance shaft. To deal with. The addition of the balance shaft increases the mass and moment of inertia, so these performance engines use short-stroke pistons and stronger structures to treat the symptoms and not the root cause to minimize these vibrations.

The firing sequence of the plane crankshaft V8 is very simple, and there is no problem that the cross crankshaft V8 and the exhaust cylinders are ignited sequentially. The cylinders that do work are always left-right-left-right-left-right-left-right..., instead of left-right-left-left-right-left-right-right like the cross axis, so there is no row For air interference problems, you can use conventional equal length exhaust manifolds to increase the power at high revolutions.

 

Cross shaft

 

Advantages: low vibration and smooth operation

 

Disadvantages: heavy weight, large inertia, exhaust interference

 

Plane axis

 

Advantages: simple structure, low inertia, good for high speed and engine response

 

Disadvantages: great vibration