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Mega-Engineering Post


#1

Thanks to another thread on here, it made me realize that a lot of people need a short (HA! No. I really will try to make this brief, but there’s lots of info here!) primer on basic car engineering and principles.

I’ll do my best.

SO! for starters, here are some things that a lot of people don’t understand about cars:

  1. Rear wheel drive vs. Front wheel drive

In any vehicle, when you accelerate, weight transfers off the front of the vehicle, and shifts to the rear. In a ‘slow’ car this is not a problem. When you are talking about a performance vehicle, it actually helps RWD cars gain traction, but hurts FWD cars. You’ve seen dragsters lifting their front wheels? This is why. Front wheel drive cars can be fast, but they cannot accelerate with the same force as rear wheel drive, and for this reason, all high-end performance cars put power to the rear tires.

Front wheel drive cars are excellent in handling, for mild power output. The steering wheels are also the drive wheels, which try to pull you in the direction you’re steering. This is fantastic even in mud or snow, especially for inexperienced drivers. Front wheel drive cars are difficult to induce oversteer with (where a car is pushed too hard into a corner, and the car tries to turn more sharply than intended, swinging the back end out).

However, because the front wheels are pulling double-duty, it can cause problems too. As the tires start to slip, you cannot turn and accelerate at the same time. This creates understeer, where a car simply goes straight instead of turning. Many drivers find this out on ice by going right into a ditch. Rear wheel drive cars are typically too light in the back, which creates a different set of handling problems when traction is limited, including uncontrollable oversteer.

  1. Engine Layout

There are three basic positions for the engine: Front, Mid, and Rear. The names seem pretty basic, but it’s important to note that mid-engine cars typically have the engine behind the passengers, ahead of the rear axle, and rear-engine cars have the engine either directly atop or behind the rear axle. It’s an important distinction, especially when it comes to weight distribution and handling characteristics.

Transverse engines (sideways) are often used when the engine is right next to the wheels it is intended to drive. The reason for that is primarily so that the power output is already going the same direction as the wheels, and thus does not require additional gears to turn its output 90 degrees, saving space and weight. This is a very popular setup for FWD cars, because there is no need for a bulky transmission tunnel to reduce passenger space. It is also fairly common in mid-engine cars, allowing use of a trunk (or boot if you prefer!) in front and back.

Longitudinal engines (lengthwise) are the most common layout in cars, for several reasons. Firstly, it allows the use of larger engines. They can be placed lower in the chassis while still leaving room for the transmission and suspension. Having the heavy parts of the car sitting low in the vehicle means that it will handle better, and not try to roll as much while cornering. When used in mid or rear-engined applications, this layout typically means there will be no rear trunk, as it is rather bulky, but offers some performance and handling improvments over transverse setups.

For small cars with modest power, transverse engines are very common - especially I4 and V6 designs, due to their compact length (width, since they’re being mounted transversely!)

Larger vehicles typically use I6, or V8 engines, in part due to power requirements, and a need to reduce noise, vibration, and harshness (but more on that later).

The classic American car layout has always been ‘front V8, Rear wheel drive’. Such a setup provides a low center of gravity for good handling, reasonable weight distribution (because the transmission is in the middle of the car), and the whole system is rather simple, in terms of mechanical engineering. Because American roads are typically wide and very straight, the emphasis is on power and passenger comfort. In the last few decades, this trend has changed. Big passenger cars have been replaced by SUVs, and smaller vehicles, most of which are powered by inline four cylinder engines, and V6. V8 engines are still common, but only in larger SUVs and trucks, along with some high-performance cars. The use of more than eight cylinders is almost unheard of, except in some pre-World War 2 cars, V10 trucks, and a single sports car.

European cars are often more innovative, preferring smaller chassis, engines, and drivetrains to maximize passenger space and handling, while keeping the vehicle size smaller overall. This is in part due to small twisty roads that are commonplace, but also because of a desire to keep fuel consumption low. Some of this trend was also forced by legislation and taxation on cars. In some places, cars were taxed based on displacement, the number of cylinders, or the size of them - which prompted car makers to use smaller engines with less cylinders overall, and inspiring great creativity.

Even when more cylinders are used, displacements are typically kept low, preferring high horsepower with modest torque. The lighter weight of these higher-performance designs allows for smoother application of power, without overwhelming the tires in acceleration, and a strong emphasis on handling over brute performance.

  1. Basic Engine Design

Any type of engine can be used to provide a given amount of power. In order to understand that, you need to first know the difference between torque and horsepower.

Torque is an engine’s ability to twist with a certain amount of force. Imagine for a moment that you have a 1 lb weight on the end of a one-foot stick. (bear with me, you metric types; the amount of weight and distance is not critical here) If you can pick it up, you are exerting one pound-foot of torque. If you increase the length of the stick, it requires more effort to pick up the same weight, so that’s why the measurement includes a distance. It’s simply a measurement of force. In the metric system, it’s talked about as Newton-meters, and for just the same reason.

Now imagine this: You have a big water wheel attached to a mill. It’s slow, only spinning about 4 RPMS, but it can pick up 3000 lbs, when you hook a 1 foot bar to the output shaft - very strong!

Now compare that to a Ford 302 V8 motor, which can produce around 300 lbs of torque. In other words, with a 1-foot bar attached, that little engine can pick up only a tenth of the weight of the water wheel without stalling.

But here’s the kicker: the water wheel is only able to lift that weight very slowly. At 4 RPMs, nothing happens very fast. The 302, on the other hand, is making that 300 lbs of torque at about 4000 rpms, so it’s happening very quickly indeed! What does this mean?

Well, horsepower is a measurement of how FAST a certain amount of work gets done. In metrics, it’s rated as Killowatts, but I’ll give you the formula for horsepower. It’s (RPM * Torque) / 5252=HP

So, if we plug in those numbers, the water wheel makes (4 * 3000) / 5252 = 2.28 horsepower.

That little 302? (4000 * 300) = 228 horsepower!

So which is stronger? If you gear the 302 down so that it can turn an output shaft at only 4 RPMs, that is a 1000X increase in its lifting power (if we take out losses for friction, anyway). Doing that means the 302 can lift 300,000 lbs of weight! Probably enough to make the whole mill spin.

That’s not even the engine’s horsepower peak; just its torque peak. At 5200 RPMs, that same engine produces a little less torque (only 280 lb-ft) but thanks to our formula, we’re getting the work done even faster - so that works out to be around 277 horsepower.

Why is any of this important?

It’s all in how you build the engine. Small displacement engines make less torque, but can spin to higher RPMs without the pistons and connecting rods flying apart. Big displacement engines make more torque, but can’t rev as high.

It’s a balancing act.

Early enginemakers didn’t get this, so they just tried lots of ideas to see what worked out. Here are some things they discovered:

Some of you may have heard of the Beast of Turin: a Fiat 28.5L (yes, nearly 30 liters displacement) inline 4 cylinder engine race car. It produced around 300 horsepower, at some pretty low RPMs. When you have cylinders the size of trash cans, it’s kind of hard to make them slam up and down at 6k! Good thing you don’t need to. And given that it’s four earth-shattering kabooms happening maybe as much as 3000 times a minute (1500 RPMS x4/2 because it’s a 4 cycle engine), it’s not hard to guess what the noise, harshness, and vibration are like. Mufflers weren’t even an option for this thing.

V16 90 degree Cadillacs took two early V8 engines, and bolted them together. Two distributors, two intakes, two carburetors, and one massive 1200 lb engine sharing a single crankshaft. Even though it had lots of problems, when it ran right, it was SMOOOOOOOOOOTH. Even at low RPMs, that many cylinders means the explosions are small, and the vibrational and torsional forces help balance each other out. It was exceptionally quiet, and could pull away from a dead stop in 4th (1:1 gear) without any difficulty.

Basically, you can take any engine layout and make big horsepower. However, it’s going to be a compromise. Here are some other important-to-know bits about engine layouts.

For inlines, 3 cylinder engines cannot be balanced. They are inherently very rough-running little paintshakers, and generally limited to being very small and weak, with lots of rubber insulators to keep them from rattling all the bolts loose in your car.

4 cylinders are a little better, but still suffer from some vibrational problems, which are aggravated when engine power output is high.

Inline six cylinder engines are inherently balanced, and quite smooth in operation. Inline eights are even better, although those are not available in the game (yet?). They were never very common, because they are so long.

By cutting an inline engine in half and mounting the cylinders in two banks sharing a single crankshaft, we get a “V” engine. It is important to note that the angle of the V is important. If you place the banks ‘flat’ at 180 degrees of rotation from each other, you wind up with a Boxer engine. The piston vibrations (mostly) cancel each other out, even in a 2 cylinder configuration. The downside is that they are very very wide, and it is difficult to fit them between suspension components in the front of a car.

For a V6 it is important to use 60 degree V, because you have a cylinder firing every 120 degrees (720/6). If you put the V angle of the engine at 90 degrees, you couldn’t make the cylinders fire ‘evenly’. It would stutter, with some ‘bangs’ being closer together, and some being farther apart. They would run well if the banks were set at 120 degrees too, but unfortunately, that makes the engine quite wide and eliminates most of the advantages of a V configuration. V12 engines run VERY smoothly at 60 degrees.

V8 engines are typically 90 degrees, and the counterweights of a typical V8 make this a very smooth running engine. Some V8 engines use a ‘flat plane crank’ which does not use counterweights. This creates more vibration, but offers some benefit to high-RPM performance due to weight savings, along with changes in the exhaust pulses and firing order.

I know this was a huge post, but I hope this helps some of you. If you have more questions, I will do my best to explain what you need to know.

Best wishes!