Types of distribution in current engines and their operation

La distribution of an engine It is in charge of keeping everything coordinated so that it works correctly. Thanks to it, the valves open and close at the right time to take in air or expel exhaust gases. Nevertheless, there are many types of distribution, depending on how the components are distributed and what system they use to move.

In this article we are going to describe how each of them are, although some have fallen into disuse. Not all engine types they have stood the test of time, being outperformed in efficiency or performance by newer, more advanced designs.

Engine Timing Operation

Before starting, it is worth reviewing the basic operation of a four-stroke combustion engine, which is the most used today. It receives this name because its cycle is made up of four phases:

• Admission: The engine takes in air and mixes it with the fuel. It occurs when the piston lowers and makes room for air, which is let in by the open intake valves.
• Compression: The engine compresses the air-fuel mixture. It occurs when the piston rises and the valves are closed to create a sealed chamber.
• Expansion: the mixture is ignited by a spark in gasoline and gas or for the high pressure diesel. This causes the explosion or expansion of the gas, which pushes the piston down again.
• Escape: the piston rises again thanks to the circular movement of the crankshaft and Rods. This pushes the exhaust gases out, through the exhaust valves that open in this phase. In this way, the combustion chamber is ready to start the cycle again.

As you have seen in the different parts of this cycle, the valves have been opening and closing to make the engine work. If the distribution did not synchronize these movements well, very serious damage would be caused. For example, if a valve were to stay open when the piston reached the top, the engine would suffer expensive damage to repair. So much so, that many times it is cheaper to buy a new engine or simply change cars.

Therefore, the distribution has to coordinate the movement of the camshaft that activates the valves, with the movement of the crankshaft that is linked to the pistons. Nothing will collide inside the cylinder and they will fulfill their function at the right time. That said, it only remains to exposeThe different types of distribution that can be grouped in various ways: depending on where the valves and the camshaft are, depending on the number of camshafts, depending on how the valves are actuated and depending on the control element that synchronizes the assembly.

Depending on the location of the camshaft and valves

Although today's engines often have valves and cams in more or less the same place, there have been various systems throughout automotive history. Let's see each one before describing the most widespread today:

Engine with distribution SV

These acronyms mean Side Valves, Side Valves in Spanish. It is a very old system and the simplest of the three. As the function of the distribution is to coordinate the camshaft with the crankshaft, it chooses to bring both parts as close as possible. Therefore, the shaft and the valves are in the block, near the bed which is where the crankshaft is.

This system fell into disuse because valves placed on the side of the cylinder did not leave much room for piston stroke and required smaller valves and/or larger combustion chambers.

OHV Distributed Engine

Its acronym means Overhead Valve, which in Spanish translates as Valves in Cylinder Head. This system leaves the camshaft in the block, but take the valves to the engine head. What a distancing of both elements which was covered with push rods and some rockers.

Furthermore, the walking distance what remains between crankshaft and camshaft allows your movement to be transmitted with ease. It is enough that the two pinions of both elements have a third party interposed or that they communicate by means of a very short chain. This leads to the low maintenance of this distribution system, which was estimated every 200.000 km. Nevertheless, stopped being used in the early 90's to be overcome in various respects by the following type of distribution.

Engine with OHC distribution

What acronyms mean Overhead Cam and that it has the camshaft and the valves in the cylinder head. That is to say, now the elements that are separated are the crankshaft and the camshaft, not the valve shaft. This reduces the number of elements of the distribution, with respect to the OHV in exchange for putting a longer belt between both components.

This is the type of distribution that has been imposed today for three reasons:

• As the communication between the camshaft and the valves is more direct, it allows a closing and opening of valves with greater precision.
• The maximum revolutions to which it can work are mayores to those of the OHV thanks to the fact that there is not a complex system of rods that would deteriorate very quickly.
• Does not have the valve clearance problem of engines with SV distribution. Therefore, it does not require excessively large chambers to house them or valves that are too small.

The only downside that these motors have is that the transmission of the movement requires a longer chain or strap and therefore requires more maintenance. Although this has been alleviated over time thanks to the use of highly resistant chains or straps.

According to the number of camshafts

An engine's timing can also be categorized based on its number of camshafts. As we have mentioned before, there are intake valves and exhaust valves. Well, its drive can be thanks to a single camshaft, for both types of valves, or two trees, each for each type. As today's engines are of the OHC type, they are often referred to as Single Over Head Cam (SOHC) or Double Over Head Cam (DOHC).

It should also be remembered that there are motors with up to four camshafts. This is because they are V or W engines, which therefore have two cylinder heads and two rows of cylinders with their respective intake and exhaust valves. To mention an example: the V8 engine of the Ferrari 488 it works thanks to four trees.

According to valve actuation

Distribution types can also be organized according to the how the valves are actuated by the camshaft. As its name suggests, it is a shaft full of cams, which are nothing more than the protuberances that push the valves to open the duct. However, this relationship still has another intermediary:

• tappet: In OHC engines, the cams push the hydraulic tappets that are attached to the valves. They serve to reduce wear and, in essence, are a simple block of a material resistant to friction. On OHV engines, since the camshaft is away from the valves, the tappets are at the lower end of the pushrods. At the other end they usually have rocker arms that finally reach the valves.
• Couch: the cams actuate rocker arms which in turn push the valve. In OHC engines they are used when the camshaft is not directly above the valves and a short distance transmission of motion is required. In these cases, it is especially necessary when it comes to an engine with a single camshaft, which has to take care of the intake valves on one side and the exhaust valves on the other.
  They can be rockers tilting u oscillating. The first one has the axis in the center and works like a swing: the cam applies the force to go up one side and therefore the other side goes down and pushes the valve. The second has the shaft at one end and the cam hits the center for the part not attached to the shaft to lower and open the valve.

According to distribution control

Here we come to the most well-known classification of distribution types. It is very common to hear how it is referred to that an engine has distribution belt o by chain. To which we are going to add a third, which is through toothed wheels, and a fourth that directly dispenses with any mechanical distribution.

belt distribution

The camshaft and crankshaft are connected by a timing belt. This has two faces: a jagged to hold on to the sprockets that have both elements at their ends and another plana which is where the idler pulleys tighten. These rollers are necessary so that the belt is well fixed to the sprockets and that it cannot slip at any time.

• Its advantage is that they generate less noise that the chains
• Its disadvantage is that you have to change them more frequently. One of the mandatory maintenance of the car that should not be forgotten.

chain distribution

The distribution chain is much stronger than the strap and therefore does not have to require maintenance throughout the useful life of the motor. Only in cases where the car reaches very high mileage is it necessary to change it or at least check it.

As in any other element of the car, it can also deteriorate due to misuse or manufacturing defects. Whether overstretched or broken, engine damage would be very serious and would require a very expensive repair.

Distribution by toothed wheels

When the distance between the crankshaft and the camshaft is short, as in OHV engines, it does not have to be necessary to use a chain or a belt. I just knowand interposes another pinion that communicates the movement. A system more typical of old engines.

Also it is possible to find some modern engines that use gear wheels. As they are of the OHC type, the distance between the shaft and the crankshaft is greater, so it is necessary to insert more sprockets. In these cases, it must be taken into account that if the number of intermediate wheels is even, the crankshaft pinion and the camshaft pinion will rotate in opposite directions. That is why they usually have three sprockets.

Engines without mechanical timing

The distribution is nothing more than connecting the movement of the components of an engine so that they move in unison. Therefore they are more moving parts with which there is load, losing power and efficiency in the process. To solve this, there are brands like Koenigsegg, which replaces the camshaft with a technology called FreeValve. It is an electronic system that actuates the valves at the right time on its own, and therefore does not require a camshaft, belt, chain or sprocket.

An engine with FreeValve can open and close valves as needed in every moment. Therefore you can delay or advance the opening of the valves, to maximize performance or efficiency, or even bypass some cylinders to save fuel. In other words, the same as other brands such as BMW, Citroën, Ford, Mercedes, Peugeot, Renault or Volkswagen, with their variable valve timing systems, but without any complex mechanical components. Or what Audi or Porsche do with their cylinder disconnect system, again without adding extra parts that could complicate its reliability.

However, it has one more addition. Can fit incredibly precisely the opening and closing of the valves, according to the actual operation of the engine, not according to its theoretical operation. Therefore, the motor will operate more efficiently and accurately in all circumstances.

Other brands that have played with this idea have been Valeo, which developed a prototype of this type of engine together lotus, Qoros, a Chinese brand that commissioned this work to Koenigsegg. Even Fiat it came even slightly closer to this concept with its Multiair engines. Although in your case only removed only one camshaft, leaving the other to run on a lifelong leash.

The other side of the coin of this type of engine without mechanical distribution is that its development has a great technical difficulty. Any failure in the electronics that coordinates the opening of the valves can result in a blunder that breaks the engine. That is why only a few brands have managed to develop engines with this system, although for the most part it has been as a mere prototype that has not made an appearance on the street.

Images – Andy_Jensen, LC Nøttaasen, Pete, h080, Ivan Radic, Les Chatfield, Nick Ares