You may have heard it. When it’s hot outside and you are driving uphill, you hear a curious rattling sound from deep within your car’s engine. If you let up on the gas, the noise stops, push the pedal down and the rattling starts up again. What you are hearing is called pre-ignition or “knocking.” It’s what happens when the air-fuel mixture in the cylinders self-ignites. The important thing to know is that if you allow knocking to occur long enough, it can damage your engine. Needless to say, you want to pay attention to it.
Causes of engine knock
Let’s take a look at how engine knock occurs. The sequence whereby air and gasoline are burned in an engine is carefully designed for maximum power and efficiency. When the air-fuel mixture is properly ignited, all is good. The problem comes when an engine is under load and hot, and the air-fuel mixture ignites itself before the spark plug fires. Essentially what happens is that there are two explosions in the cylinders instead of one. This means that two flame fronts are created and they blast into each other. That’s what the knocking sound is, the flame fronts hitting each other.
Low octane gas
The octane rating of the gasoline in your car will affect the engine’s tendency to knock. Here’s why: The octane rating of gasoline is a measure of how easily it ignites. A lower octane gas ignites relatively easily and a higher octane doesn’t. Your car’s engine is designed to run on gas with a certain octane rating. This rating is set by the manufacturer of the car and is noted in the owner’s manual. If you want to avoid knocking, stick with the octane gas that the car maker recommends.
The way that knocking can damage an engine is via the flame fronts hitting each other, as we explained before. Believe it or not, the pressure wave that is produced by two flame fronts hitting each other is enormous. This pressure is not high enough to damage the iron cylinder walls but it can damage the pistons. This is because pistons are usually made of aluminium which is far softer than iron. We asked the service manager at Newark Chrysler of Newark, a local Chrysler, Dodge, Jeep, Ram dealer in Newark, DE, what exactly happens when an engine knocks to long. He said it’s simple, it actually blows holes in the top of the pistons.
Deposits on the cylinder head
Knocking can be caused by carbon deposits inside the cylinder too. Here’s how: When a mixture of gasoline and air is ignited, there is a chemical reaction that produces CO2 (carbon dioxide) and H2O (water). Well, that’s the theory anyway. In practice, small amounts of other gases and soot are produced, most of which are blown out in the exhaust. The tiny amount of soot that stays behind eventually clumps together and forms what are called “carbon deposits.” These are spots of carbon that glow red hot in the engine. When this happens, it can ignite the gasoline air mixture before the spark plug does. The result is flame fronts hitting each other and this results in knocking.
The only solution to this is to remove the carbon deposits and this is work that a mechanic should do. In many cases, a mechanic can pour a special fluid in the intake manifold while the engine is running and this will blow the carbon deposits off the cylinder head. Other times, the engine needs to be disassembled and the cylinders manually cleaned.
The wrong spark plugs
Using the incorrect spark plugs for a specific engine is a common problem, according to Edmunds.com. Since the spark plugs operate under precise conditions, the wrong plugs can mess up the combustion cycle and cause knocking. The solution to this is obvious. Remove the “wrong spark plugs” and put in the right ones.
In summary, if you hear your engine knocking often, you should attend to it soon. In many cases, a trip to your local mechanic is the best option. Do it before any damage occurs.