I’m currently reading A Scanner Darkly–excellent book. There’s a scene where this car these men are in goes haywire. One guy mentions something about RPMs.
When I think of those, it’s revolutions per minute on a record player. I have no idea what that means in terms of a car engine. Please give me your most detailed car knowledge so I can understand it better.
Thanks!
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Okay. So imagine this: a car engine is a contained series of mini bombs. These bombs explode against a cylinder of metal, called a piston. When this explosion happens, the piston moves down. And because of how these pistons are set up, and how the explosions are timed, they go off in a pattern, based on how many cylinders the engine has.
Now, these pistons are connected to something called a crankshaft. Think of it like a rod, but with segments that go up and down to connect to the pistons. When each of the explosions go off that I mentioned above, it helps to turn this crankshaft.
Specifically, regarding what you are asking, RPM, it’s referring to how many times in a minute that crankshaft rotates.
This is going to be very generalized (and is limited to the typical gasoline engine in a car; there are variations on this for other fuel types and engine types), but here’s how I’d explain it;
An internal combustion engine consists of a large chunk of metal known as a “block”. That block has a set of cylindrical holes drilled through it. One hole runs the length of the block, and houses a special rod known as a “crankshaft”. There are a number of other holes, drilled perpendicular to that one, known as “cylinders” (the number of which varies based on engine design; but it’s most commonly 4, 6, or 8 in a car, with the general rule that more cylinders equals more power)
The crankshaft is not a straight rod. It has sections that are offset from each other. I guess that you’ve never ridden a bicycle, but perhaps you’ve felt the pedals and the metal shaft that connects them? And you might know that it’s made so that when one pedal is pushed down, the other is pushed up (and vice versa, as the whole thing rotates around the axle)? And by alternating which pedal is pushed down, you turn the sprocket/chain? A crankshaft is basically like that.
In each of the cylinders, rides a cylindrical chunk of metal known as a “piston”. The piston is made such that it’s diameter is almost the same as the diameter of the cylinder (minus a little bit of space for a seal and to allow a cushion of lubricating oil between it and the cylinder walls)
Each piston is connected to the crankshaft by a “connecting rod”. The connections of the rod to the piston and crankshaft are able to rotate/hinge (the way the pedals on a bicycle can spin freely).
At the top of the block, above the cylinders, is mounted a “cylinder head”. This is another specially designed chunk of metal that contains a set of valves that allow for air and fuel mixture to enter the cylinder, and for exhaust to exit the cylinder. It’s also the entry point for the spark plugs that ignite the air/fuel mixture.
Each piston/cylinder and its associated valves goes through the following “four-stroke” cycle:
Intake – The piston is moving downwards, and the intake valve is open. This causes a vacuum in the cylinder, that draws in the air/fuel mixture.
Compression – Once the piston reaches the bottom, it begins to travel back upwards (the crankshaft has made a rotation, and it is pushing the piston back up) Simultaneously, the intake valve is closed. This causes the air/fuel mixture to compress and increase in pressure. This is necessary for the ignition in the next step.
Power – Once the piston is at the top of its travel, the spark plug creates a spark that ignites the air/fuel mixture, causing a small explosion. The energy of this explosion forces the piston back downwards, which forces the crankshaft to rotate again.
Exhaust – The piston travels back upwards again, while the exhaust valve opens. This forces the exhaust of the explosion back out of the cylinder.
Stroke 4 sets us up for stroke 1 to happen again. And on and on.
Each of the cylinders and their associated valves and spark plugs, is timed/coordinated very delicately, so that some pistons are moving down and some are moving up at all times, and none of them are “fighting” against each other in turning the crankshaft.
This results in a continuously smoothly turning crankshaft.
The speed at which the crankshaft is rotating is the engine RPM. And yes, it’s the same unit; revolutions per minute. But while a record player spins at either 33 1/3, 45, or 78 RPM, a vehicle crankshaft spins at 600-900 RPM at “idle” (when the car isn’t moving) and to 1000 or more when the vehicle is accelerating (with some high end engines reaching around 6000 RPM)
The rotation of that crankshaft is transmitted to the wheels of the vehicle through the “transmission”. But that’s an entirely different post.
Imagine a syringe, like you would use to give a child medicine. It has a rod inside of a larger tube. When you push the plunger, air is pushed out. When you pull the plunger up, air is sucked in. Imagine you draw air into the syringe and then stick your finger over it to prevent air coming out. When you press the plunger and squeeze the air, you can feel it pushing against your finger, but it won’t escape if you have your finger on tightly. If you let go of the plunger (while your finger is still capling the syringe) the plunger will move back out automatically, this is due to the air pressure you created when you pushed in the plunger. This demonstrates how we use air pressure to create movement.
In a car, we do basically the exact same thing. The only notable difference is that we have the plunger on the bottom and the air comes in from the top. Fuel and air are pulled into the tube (a.k.a., the aptly named “cylinder”) by the plunger (a.k.a, the “piston”) in the car. The cylinder is then sealed and this fuel-air mixture is compressed by the engine moving the piston up. When the piston reaches the top of the cylinder, we use a spark to ignite the fuel-air mixture. This creates an explosion that moves the piston back down with considerably more force than we used to compress the fuel-air mixture.
Thisbwhole cycle takes four movements of the piston. One down, one up, one down, one up. We call each stage a different name. In order, they are: “Suck, Squeeze, Bang, and Blow” which amounts to “sucking” in the air (and fuel) with a downward piston movement, “Squeezing” the air in an upward piston movement, “Bang” is igniting the fuel-air mixture amd creating an explosion that drives the piston downward, then “Blowing” out the exhaust with an upward piston movement that expels the waste products (the exhaust). You’ll notice that there are four stages and only one stage creates power. In general, engines will operate with more than one cylinder and will be timed such that each cylinder is creating power while the others are in one of the other stages. This creates a smoother feel and less vibration than if they all went “Bang” at once.
I’ll leave it to others to explain a crankshaft to a blind person, I’m not that imaginative unfortunately, but it basically converts that up-and-down piston motion into rotational motion. It connects all of the pistons in such a way that we use a little of the “Bang” energy of one piston to create the movement required for the “Suck”, “Squeeze”, and “Blow” portions of the cycle for the other cylinders. This lets the engine create a self-sustaining cycle to keep itself running.
Additionally, the crankshaft is connected via clever gears to the wheels, which lets the engine turn the wheels and move the car.
RPM is just how many times one piston completes a full four-stage cycle in a minute, exactly like the record player you mentioned.
The top post sums it up nicely the only thing I would add is if you know how a bicycle works it is very similar except the pedals are the pistons and your feet are the “explosion “. I don’t know if you have ever used a stationary bike or know how one works so this may all be a pointless reply.
Think of two soup cans, one slightly smaller in diameter than the other, so that it fits inside the larger one snugly. Then imagine you drilled a hole inside the end of the larger one, and squirted someting flammable inside. What happens when you ignite the mixture with the smaller can stuffed in the larger one? The small can goes flying.
But what if you attached the under side of the small can to the end of a crank handle (like the handle used to wind down windows in older cars) with a rod, so that the small can didn’t fly away, but it almost came out of the larger can, and then went back inside as the crank came back around? That’s a piston. Each time the piston returns to the top of the larger can, you squirt in some fuel and ignite it, pushing the piston back down. Each time the piston is forced out by the explosion, it turns the crank.
Then, you connect the pivot point of the crank to the center of a wheel with a shaft. Now the wheel spins every time the crank spins. This is a very rudimentary example of how a car works.
You’re right, RPM just refers to rotations. In this case, instead of the rotation of a record, it’s how many times the crank rotates in a minute. Knowing RPM is useful for certain things. Each time the fuel burns, more power is added to the crank, so the more often it happens, the more power the engine produces. An engine running at 5000 RPM will produce far more power than the same engine running at 1000 RPM because more work per unit of time is being fed into the system.
It gets so very complicated, but that’s the gist of it. I apologize if my descriptive skills failed to get the idea across.