Powertrain: POWER! & How to Manage it
The powertrain of the car is what actually makes the car move. The train starts in the fuel tank, fuel goes to the injectors, fuel injectors spray fuel into the air that goes into the cylinder, combustion occurs, pistons move, and ends with the wheels doing burnouts. A quick flowchart shows how the fuel travels through the system and makes the car move.
Now, when you’re manufacturer has a warranty for your powertrain, it typically includes everything from the engine block to the axles. Just be sure you read the fine print to see exactly what’s covered.
Most cars have their transmission separate from the engine. While it is a more bulky setup, the transmission can take more load. Motorcycles have the transmission encased with the engine to reduce size and weight. The tradeoff is if you put the same size gears in a normal size car, they would be ruined very quickly due to the higher loads in street cars.
Going from top to bottom, we’ll start with the injectors. The injectors introduce a predetermined amount of fuel into the cylinder. This method is called fuel injection and is what most of today’s gasoline-powered engines use. Fuel injection requires the use of an Engine Control Unit (ECU) that monitors and controls everything the engine is doing. This is what replaced carburetors that controlled the engine mechanically. Harley-Davidsons, classic muscle, and other similar vehicles used carburetors.
Next up we have the valves. Each cylinder has two types: an intake valve and an exhaust valve. The intake valve takes in clean air from the plenum and puts it in the cylinder. This will be the air used for the combustion of the fuel brought in by the injectors. The exhaust valve takes the exhaust gases left over from the combustion and empties the gas into the headers. These valves are controlled by the cam shafts which sit in the head of the engine. It’s important that these valves have good seals or else the combustion won’t be contained in the cylinder, bringing your power down.
Cam shafts control the timing with which the valves open. They are linked via a timing chain to the crankshaft (which is the output mechanism for the energy produced). On these camshafts are little notches (or lobes) that will push the valves open. The way these are positioned in relation to the pistons is critical: too far one way could mean air isn’t brought in on time or the exhaust isn’t fully cleared before firing. Our engine uses overhead cams, meaning there are two cams, one for each type of valve. V engines (V6, V8, etc.,) have one cam going down the middle.
Pistons and rods are probably the most commonly known part of the engine, if not the easiest to visualize. The piston head is the solid cylinder that moves up and down in the engine cylinder – this is the guy that is exposed to the combustion and has the most amount of friction and heat subjected to it. He’s connected to the rod, which is connected to the crank shaft. These rods need to be extremely strong to withstand intense amounts of heat, loads, and accelerations. When the RPM gauge (or tachometer) reads 6,000 RPM, that means that every 60 seconds each piston has gone up and come back down 6,000 times – that’s 100 times a second! Our car redlines at 13,500, a blinding 225 cycles per second. Don’t do that in your Yaris, please.
The crankshaft is what actually connects the engine to the transmission (in a roundabout way). From here, the transmission gears determine the speed at which the wheels spin. Crankshafts also determine the displacement of the engine. An engine’s displacement is calculated by measuring the piston head area times the vertical travel of said piston times the number of pistons. So more travel = more displacement. Normally measured in liters for cars and cc’s (cubic centimeters) for bikes, this is one of the many stats you’ll find gentlemen bickering about. “Bruh, my Yaris has a 1.5 litter in it! It’ll beat anything out there!” “Yea. Sure. OK buddy. *fires up 6.2L Corvette*” “Pees pants”. Per FSAE rules as of 2016, we are only allowed to have at most 610 cc’s of displacement. Our Suzuki GSXR-600 engine barely makes the cut.
Between the differential and engine is the transmission. This guy is where all the gears are and determines how fast you can go. Gearing is extremely important in racing because you always want the engine to be spinning in the power band. The size of the gears affects how fast the engine spins in relation to the driven wheels. Longer/bigger gears = faster top speed but lower acceleration, shorter/smaller gears = slower top speed but faster acceleration. Deciding what you want your car to be geared for is very tricky business, as most courses have both long straights and tight turns (unless you’re NASCAR, in which case you’re stuck doing a long left-hand turn). Making sure the gears work with one another is important as well. You want to have your shift points just past peak power, but you don’t want the engine to “fall” too low in RPM when going to the next gear because you’ll spend extra time getting back to the power-making RPMs. Final drives (what is actually connected to the drive shaft) will adjust the gear ratios as a whole, i.e. a longer final drive means everything has a higher top speed but slower acceleration (this is much much easier to adjust at the track than rebuilding a transmission to change just one gear).
The differential takes the power output from the transmission and sends it to the wheels. There are 3 types: open/unlocked, limited slip, and locked/closed.
Open diffs have nothing to stop a tire from spinning when it breaks lose. Older Mazda Miatas, front-wheel drive cars, cheap RC cars, and strollers are all examples of open diffs. These are great for turning sharply, as the inside and outside tires are spinning at different speeds. An open diff allows that to happen, so the tires aren’t fighting each other and pushing the car out. These are great for cars that have no power, cheap RC cars, and for doing one-tire burnouts (exciting and macho, I know).
Limited-slip diffs are just that: they let the tires “slip” slightly in relation to each other. How quickly they lock and under what conditions is totally configurable, meaning the tuner has control over when the tires start spinning together. There are two settings that are important with this type of diff: accel and decal. Under acceleration, you can set it so the tires lock sooner, meaning the power is being applied evenly to both tires. Under deceleration (braking), you can set it so the tires lock a little later, allowing the difference in speed between the tires to be greater and allowing for tighter turns. Again, this all depends on the track you have to work with. Tighter tracks may favor a more open setup while faster and longer tracks favor a closed setup.
Locked diffs don’t move at all – they keep the tires spinning together all the time. This is great for high-powered drag cars that only go in a straight line really fast. Any difference in tire speeds can send those guys careening into a wall at over 200 mph. These locked diffs are literally the worst thing for turning, though. Because the tires want to spin together and not at separate rates, they push the car through the turn, meaning the turning tires are slipping and the car isn’t turning as tight as you’re telling it to.
The last components are the axles and tires. The axles go out from the diffs to the hubs, which are bolted to the wheels, which are holding the tires on (wheels are the metal part, tires are rubber). While the wheels can’t do a whole lot in terms of performance and tuning, the tire choice has a profound effect on the performance of the car. (Before y’all get your knickers in a twist, yes, the wheels do effect performance. But they aren’t a “tunable” item. Material choice and design is just as important here as it is everywhere else on the car, but you don’t normally have a wheel for dry conditions and one for wet.) In on-road racing, there are three main categories of tire: slicks, intermediates, and wets.
Slicks have no tread on them whatsoever. These are great for dry, clean conditions. They offer the most amount of traction of the three types. However, as soon as the track gets dirty or wet, the tires instantly lose grip. As the tire rolls over water on the track, the water can’t escape from underneath and acts like a barrier between the road and tire. This is how your normal Yaris hydroplanes on the highway during heavy rain, but for more reasons.
Wets are for use only in very wet conditions. These tires have treads on them to allow the water on the track to escape through when the tire rolls over. Because water is in incompressible fluid, it squirts out the side through the tread grooves, allowing the rest of the tire to maintain contact with the road. While great for supersoaker weather, these tires heat up extremely fast on dry pavement, essentially making it like driving on ice (too hot = less grip for any tire).
Intermediates are just that: an intermediate step between a slick and a wet tire. These have many variations that correspond to the dampness of the track and what the customer wants. FSAE does not permit teams to make their own intermediates, so most teams just have slicks and wets. All other championships like the World Endurance Championship, F1, Touring, and others will have all three types. Some teams will even have their own tread patterns and compounds.
Speaking of compounds, each tire type has a different compound. This is to make sure the tire heats up properly when driven in the right conditions. Wets are designed to heat up faster than slicks because the constant application of cool water will keep the tires cool. Without that water, wet tires heat up too much and become gooey to the point of melting off. Some compounds heat up more than others, some offer more grip at lower temperatures, and others use dark matter to bend the universe and make you go faster. Choosing the right compound is just another one of the many decisions teams need to make when making purchases and designing their car.
Now that the engine has made all of this energy, not all of it goes to the wheels. Most of the energy is “lost” to heat, meaning it’s unusable. This heat sits in the engine and makes it hotter and hotter until the pistons seize. To combat this, radiators are used to let the heat escape into the atmosphere. Water or coolant circulates through the engine and brings the heat out to the radiator. As air passes over the fins of the radiator, the water cools back down and is sent back to the engine, where it’s reheated and the cycle repeats. There is a high temperature where the engine is designed to operate at, so a radiator that works too well is a thing. There are many types of radiators that I won’t go into here in the interest of keeping this shorter than a novel, but Wikipedia is always fun to read. If there’s enough interest, I can also write about a specific component more in depth.
Woohoo! You’ve made it through another long and arduous post about a car system! And we’re getting close to the end! I wonder what I’ll do after this… Maybe posts about how to not hit cones or how to do burnouts or something like that… But before we can get there, we still have the electronics system, cockpit, controls and safety system, and data logging to learn about. If you have any questions about a system or want a more in-depth explanation, shoot me an email and I’ll see what engine gurus tell me. Until next time!
- K
