Taylor Made: Feathers
When I was ten years old or so, my grandpa (rest his soul) decided it was time for me to learn to drive a stick shift. We climbed into a blue ’87 Jeep Comanche (fun fact: Jeep was a division of American Motor Corporation until 1987, so this was one of the last made by AMC instead of Chrysler) and he told me I was going to “put the clutch in and start the car, then let the clutch back out while giving it a little gas.” As he explained this, he mimed the process with his hands, letting up on an imaginary clutch with his left while slowly pressing an accelerator with his right. I pictured doing what he described, fired up the truck, and promptly stalled the engine.
“You have to feather the clutch,” he said, leaving it up to me to decipher this secret of the universe. But after a couple dozen attempts that all ended in the engine quitting, it became clear that I wasn’t going to feather anything that day. He finally took over and drove off, smooth as you please, while I watched, exasperated. But the next time I got in that truck, I drove. And, although I didn’t recognize it at the time, that summer marked the true beginning of my interest in cars (I was already asking about engines thanks to a particularly charismatic carbureted straight-six in a late ‘70s speedboat, but that’s another story). You can read about my grandpa’s experiences as a Ford engineer here.
Spring for me now means the start of driving season (and, consequently, the start of what-mod-do-I-need-to-do-next season; for an example, see this blog), although spring was slow to show her sweet side this year. Nonetheless, in the spirit of warm weather automotives, let’s talk our way through the basics of car engines.
When you start the car, you’re completing a circuit that sends current to the starter motor. In the age of cars with keyed ignitions, which seems to now be markedly on the decline, turning the key would physically connect the circuit. The starter motor throws a gear onto the engine’s flywheel (or flexplate, if it’s an automatic transmission) and spins it, making the engine “turn over.” We use this phrase because the flywheel turns the crank, making the engine’s internals rotate.
Just what are these internals, you ask? The engine block is comprised of a crankcase, which houses the aforementioned crankshaft, and a block of cylinders, robust chambers within which chemical energy is converted to mechanical energy. Each cylinder contains a piston, a cylindrical-shaped drum pinned to a rod (aptly called the connecting rod). The piston’s vertical motion in the cylinder actuates the connecting rod, which pushes and pulls on the crankshaft. Although the process of starting the engine uses the flywheel to turn the crankshaft, power travels in the opposite direction once the engine is running, with the crankshaft driving the flywheel. The clutch, when engaged, connects by pressure to the flywheel to send torque through the transmission to the wheels.
The pistons need lots of energy to rotate the crank, hence the combustion aspect of internal combustion engines. Unlike external combustion engines, for which combustion happens outside the engine, ICEs see combustion happen in their cylinders. This brings us to the four-stroke cycle, which describes the energy conversion process taking place. During the intake stroke, the piston moves downward in the cylinder—toward bottom dead center, if you want to get technical. The expansion of volume between the piston head and cylinder head results in a pressure drop; note that the piston makes a gas-tight seal between itself and the cylinder wall so that the changing pressure and volume are not compromised. An intake valve is timed to open during this stroke (a.k.a. movement of the piston through its range), allowing air to be sucked into the expanding space. What actually enters is a mixture of air and fuel, as fuel injectors (in modern engines) will have sprayed gasoline into the fresh air coming from the intake manifold. The intake valve then closes, sealing the mixture inside the cylinder. During the combustion (second) stroke, the piston moves back to top dead center, compressing the mixture. Each cylinder head has a spark plug sticking through it, and each spark plug has two metal tines with a tiny gap between them, across which current will jump when the necessary voltage (think tens of thousands of volts) is reached. The current jumping the gap creates the spark that ignites the air-fuel mixture. The explosion forces the piston back down in the power stroke, named thus because the piston’s powerful downforce turns the crankshaft, sending energy to the flywheel and so on. Lastly comes the exhaust stroke, during which the piston again returns to TDC while simultaneously the exhaust valve opens, allowing the burnt mixture to be pushed out of the chamber and into the exhaust manifold.
The intake and exhaust valves are actuated by cams, specially designed wedges that sit along the camshaft. As the camshaft rotates, each cam pushes on its corresponding spring-loaded valve, depressing it into its cylinder to briefly open. The angles of the cams relative to the longitudinal axis of the shaft are offset from one another so that the valves open and close at different times as the shaft makes one complete rotation. For the engine to function optimally, it is crucial that the timing of the pistons moving up and down lines up just right with the timing of the valves opening and closing. The camshafts and crankshaft are timed (i.e. connected) to one another using a timing belt or chain.
A lot of engineering goes into how an engine’s set of cylinders are arranged. Common configurations include inline, in which the cylinders sit in a single row (also called straight); flat, in which cylinders oppose each other at 180 degrees with the crankshaft between them (also called boxer or horizontally opposed); and V, in which two rows of cylinders oppose each other at an angle to create a V shape. An inline configuration might sit at a slant instead of upright; it might also sit transverse, meaning perpendicular to the direction of travel (common for front wheel drive cars). While even-numbered configurations are much more common (in general, symmetry makes for easier balancing), specialized arrangements like an inline-five, which is more compact than a straight-six and smoother than an inline-four, are also used. And if you really want to have some fun, there’s always the rotary engine (shout out to the RX-7), where a rotor spins within a housing in place of traditional pistons and cylinders, progressing through the same elements of the four-stroke cycle as it makes one complete rotation.
These configurations address the issue of balancing in different ways. The rotary engine, notoriously hard to service, is also famously smooth-running because it produces torque without pistons. Pistons require a lot of balancing—each one produces massive downward force during the power stroke that has to be counteracted. The crankshaft is balanced against the pistons using counterweights, and design factors like the arrangement of the cylinders and the firing order of the pistons help to balance them against each other. For example, inline-fours typically fire 1-3-4-2 to best separate each power stroke. Higher performance engines often make use of additional components like balancing shafts.
For all my words, I’ve only skimmed the surface; diesel engines, which fire by compression rather than spark, are their own rabbit hole. And what about the differences among fuel octane levels (like how, for instance, using 93 over 87 can decrease knocking)? And then there’s air-fuel ratios and their role in the engine’s performance (run too lean and overheat, too rich and reduce combustion); let’s not even get started on turbocharging. There’s a whole world of engineering and design within that apparent mess of puzzle pieces in the engine bay, and with summer just around the corner, what better time to stick a toe in the water, so to speak, and explore it?