Have you ever looked at the various parts on a tractor or combine and asked, “just how does it work?”
I will attempt to answer those questions in a series of articles because the more we understand how things function, the better off we’ll be at maintaining them or fixing them.
Let’s talk about the engine first. This thing sitting under the hood can convert a small amount of fuel into enough power to pull a 60 foot cultivator through the dirt, complete with 10 inch shovels.
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What happens inside that engine?
It all starts with an explosion, or many explosions. Let’s call it the big bang.
We need to harness this bang to make it work for us. We put it in a cylinder and put a piston in that cylinder. When all that energy is released by the bang, it pushes down on the piston. This piston is connected to the crankshaft from which we get our power.
Each piston will fire about 15 times every second at 2,000 engine r.p.m. Here is a list of all the things that have to happen during that 1/15 of a second:
- The big bang pushes the piston down. Both valves are closed. This is the power stroke.
- When the piston moves back up, the exhaust valve opens and all the burned gases blow out.
- The exhaust valve then closes and the intake valve opens. On its way down the piston sucks in the air, in the case of a diesel, and a mixture of gasoline and air, in the case of a gasoline engine.
- When the piston moves up, it compresses this mixture and when it reaches the top, the mixture is ignited or the fuel is injected.
How important is the timing?
We are dealing with high speeds and everything must happen in a split second.
- The timing of the fuel injection or spark must be exactly right and is usually about 10 degrees before top dead centre.
You want the big bang to happen just after top dead centre. The fuel must ignite a little before that occurs so that by the time the explosion has built full power, the crankshaft will have travelled about 10 degrees of its 360 degree rotation. Remember, this happens about 2,000 times in the next minute.
If the timing is too early, the engine will produce a pinging sound and in time a piston will burn. The engine will also run hotter.
If it’s too late, horsepower will be lost and exhaust will probably be too hot, which could burn the exhaust valve.
To adjust the timing you should be able to find what you need from the machine’s manual or through various sources on the internet. If need be, call the dealer.
Diesel engines probably have a swirl chamber on top of the piston. The injector sprays into this chamber and helps mix the fuel and the compressed air for an even burn.
If timing is too early, burn marks can be seen on top of the piston and cylinder walls will be scored.
You wouldn’t want to put it back together after the money and effort of a rebuild without ensuring the timing was right. After all, this may have been the reason for the overhaul in the first place.
If I succeeded in scaring you, good. This is not something to mess with unless you know what you are doing or have a keen interest in getting it right.
Valve timing is important. If the engine turns over until the exhaust valve just starts closing and the intake valve just starts to open, they will come to a spot where they just meet each other in passing. That is the reference point for the camshaft.
The manual will indicate where the crankshaft should be at this point. If it’s not, adjust it to factory specifications.
Valve timing has a lot to do with where the engine runs most efficiently. For a car or pick-up truck, it is usually around 100 km-h in its highest gear.
For a race car or truck, the racing cam changes the whole picture because the engine will be used for a goal other than fuel efficiency at highway speeds and reasonable low-end torque for getting around in the city.
For tractors and combines, it would be at the machine’s rated loaded engine speed because that is the only place you want that machine to be most of the time.
There is another form of timing to keep in mind everyday: maintenance. When you consider how often the big bang happens, and with such precision, it becomes obvious why it’s important to make sure oil and coolant are clean and not compromised by time, heat and impurities and why the machine must breathe clean air through a regularly serviced filter.
Henry Guenter is a former service manager for Massey Ferguson. Got an equipment question? We’ll do our best to get an answer for you. Contact insidemachines@producer.com.
