Tractor PullDear Old Mechanic;

My husband and I went to an antique machinery show recently and watched the antique tractor pulls--well at least some of it.  I saw how the weight on the sled moved foreward, making the tractors pull more of a load the further they went.  My question is about one red tractor (sorry, I don’t know their names) that seemed different.  Not so much different looking, but it had a different sound and seemed to fly down the track when compared to most of the other tractors.  That tractor also seemed much quieter than the others, even though it had a big chrome exhaust pipe sticking out of the top.  It seemed to buzz or purr, where most of the other tractors had a popping or roaring sound.  Some of them actually hurt my ears.  Can you tell me why that one sounded so different and seemed so fast.

There was one other green tractor that seemed somewhat quiet.  It was not as loud as most of the others, but still was louder than the red one.  I know this sounds like a silly question, but when I ask my husband, he got way too technical and used words like "cubic cylinders" and "charging turbo things.I got lost.

Why do some of the tractors sound the way they do--different than the others?  And for your information I am not blond, just call me Not Educated About Tractors.


Dear Not Educated;

What I will be trying to do is condense what a trade school takes almost two years to teach into a one-thousand word newspaper column.  This will take several edits.

Engine Block

To begin with, we burn either gasoline or diesel fuel with air to get mechanical energy to move our cars, trucks and tractors--hereafter referred to as "vehicles.There are round, capped-up holes in the engine called cylinders, where we burn the fuel with air.  These cylinders are measured in cubic inches (or liters).  The bigger the cylinder, the more fuel we can put in there to burn and  produce power.  Some of the green tractors you saw were old John Deere tractors.  They only had two cylinders, but they were big cylinders to produce the power necessary to pull farm implements or, when in competition against other tractors, to pull the weighted sled.  With only two big cylinders, these were the ones you heard mking popping sounds.  Some of the red tractors and all of the orange ones there had four cylinders.  Although their cylinders were smaller in size (cubic inches), there were more of them to produce the power needed to pull the weighted sled.  With more smaller cylinders, there was less of a popping sound and more of a roar.  Some of the bigger red tractors and one dark green one had six cylinders with less cubic inches but more cylinders to produce the power needed to pull the sled.  The six cylinder engines were quieter, as their roar tended to blend together.

Where do all of these noises come from?  After we burn fuel and air in a cylinder, we need to clear it out so that we can put in fresh fuel and air in to burn and produce more power.  To do that, we open a valve in one end of the cylinder to get the burned fuel (exhaust) out.  Since there is still a little energy in the cylinder when we open the valve, it makes that popping sound.  With a two-cylinder John Deere engine, there is a "pop - pop" sound.  With a four-cylinder engine, there are smaller "pop - pop - pop - pop" sounds which sort of run together and sound like a roar.  With a six-cylinder engine, the six smaller pops really run together.  This accounts for some of the different sounds.

TurbochargerIf we could cram more fuel and air into a small cylinder and burn it, then it would put out more power because a small cylinder could act like a big cylinder. To cram more fuel and air in a cylinder, we need some sort of a pump or fan.  At this point, someone said, "Let's use that little bit of that left-over energy that makes a popping sound when we open the valve.  What we will do is put a fan where that left-over energy will turn the fan.  We will put the fan on a shaft and another fan on the other end of the shaft.  When the left-over engergy turns the first fan, it will also turn the other fan, and we can use that other fan to blow more fuel and air into the cylinder.That two-fan mechanism is called a "Turbocharger."  A funny thing happens when you put the first fan in front of the exhaust from the cylinder; the blades of the fan tend to muffle the sound.  If that fan is muffling the six sounds from a six-cylinder engine, it is going to produce a sound similar to what you said was a “…buzz or purr.”  That also means that more fuel and air was going into all sx cylinders to produce more power.  Yes, it seemed like that tractor could fly down the track compared to the other tractors.  I guess you left before they put enough extra weight on the pull sled to cause that tractor to only make it half way down the track.

This column is an oversimplification of how an engine works.  There are also intake-valves and mechanical components to open and close the valves at the right time.  Add to that the pistons moving up and down in the cylinders to transmit power to the crankshaft that then transmits that power through the transmission to the wheels.  As I said in the beginning, to be an entry-level technician (formerly known as a mechanic), it takes almost two years of full-time education at a good trade school.

If this didn’t help, please let me know and I'll try again.

Herr Professor Nuzanbolts