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Shop Class: Powertrain Placement

Where’d They Put the Engine?

Alex Steele
Oct 22, 2019
Engines, transmissions, differentials, and drive axle(s) are universal components common to pretty much all mass-produced internal-combustion automobiles, but an assortment of powertrain configurations has long been part of automotive design.
The engine produces the mechanical power. The transmission uses variable gearing of different ratios to make the power usable at the street, under a wide range of load and speed demands. A differential allows the drive wheels to rotate at different speeds, which permits seamless turning. It also provides additional gear reduction, and, in certain applications, redirects the rotational force from one direction to the other (ex. propeller shaft front to rear, axleshafts side to side).
This is what gets us down the road. But there's a multitude of setups that accomplish the same base task but may augment a vehicle's driveability to its designated use.
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In the Beginning
Designating "the first automobile" often kicks off a debate, but for our purposes the award goes to the Benz Patent-Motorwagen (Patent Motor Car, model no. 1) built by Karl Benz; German patent number 37435, dated January 29, 1886.
Yeah, it was only a three-wheeler (two rear), but it was powered by the efficient, gasoline-fueled, four-stroke Otto Cycle engine, joined by a primitive transmission to a differential that drove the wheels independently.
Sound familiar? There are about 1 billion passenger cars on our planet today based on the same root powertrain.
Back to the subject—the Patent-Motorwagen features a horizontally mounted rear engine with rear-wheel drive. At the time, that was probably the only way to make it work. We don't believe Karl Benz was contemplating engine weight over the rear wheels for extra grip at a drag race. Since then, it's a different story.

Engine Configuration

The Patent-Motorwagen's four-stroke is considered a horizontal engine, meaning the piston travels parallel to the street. A vertical engine is just the opposite, piston up and down perpendicular to the pavement. Horizontal can create a lower center of gravity for improved handling, while vertical is often more practical to fit under the hood.
-- Inline, Straight, or I-
You're not going to find too many cars running a one-cylinder engine, and a common setup is an inline or straight multi-cylinder. The majority of these are vertical inline-four (I-4), pulling fuel-efficient, front-wheel-drive sedans. The four pistons travel up and down perpendicular to the road, with the crankshaft anchored at the bottom.
The vertical inline-six (I-6) engine comprised a significant portion of U.S. production for years, phased out mostly due to engine length that wouldn't fit in smaller cars and crash-safety considerations. High-displacement straight-six diesel truck engines continue to hold their own, and gas I-6s may be making a comeback. The advantages are inherent engine balance (which reduces vibration) and cost: only one cylinder head, two cams, one catalytic converter, as compared to two of each on a comparable V-6.
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-- Flat / Boxer / Horizontally Opposed
In 1896, Karl Benz came out with the horizontally opposed cylinder concept. Like the Patent-Motorwagen's horizontal engine, a horizontally opposed mill has pistons that travel parallel to the road, but with a second piston running the opposite direction off the same crankshaft. The benefits include more power and smoother operation.
The layout doesn't sell in high numbers in current car production, but the design is still used by Porsche in the flat-six 911, Boxster, and Cayman, as well as the flat-four 718 Boxster and Cayman. Subaru also boasts the lower center of gravity its flat-four and flat-six engines provide. Also, we have to mention the air-cooled flat-four in the back-end of the original VW Beetle.
To avoid anybody getting bent out of shape on this one, here's the lowdown on the difference between a flat engine and a boxer.
Both are "flat" engines with horizontally opposed pistons and cylinders, just like we could call a 180-degree V-engine flat. The boxer difference is at the crankshaft. The crankpin or crank journal is where the piston and connecting rod are attached to the crankshaft. Typically, opposing cylinders share the same side by side journals and rotate in unison with each other. A boxer crankshaft has independent journals for opposing cylinders, positioned opposite each other (180 degrees).
Opposing piston in-and-out travel resembles a boxer punching, while they cancel out a degree of each other's enacted vibration. Most flat engines are boxers.
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-- V or Vee
The V design employs two cylinder banks sharing a common crankshaft, like a flat, but this time the cylinders are angled vertically at various degrees. Viewed from the front of the crankshaft, it's a V shape.
The whole idea was to make a bigger engine smaller. Picture an inline-eight with each cylinder one after the other in a straight line. Then take the second, fourth, sixth, and eighth cylinder and put them on the other side of the crankshaft at a 90-degree angle. This yields two banks of four cylinders, greatly reducing engine length.
The angle of the V varies by application, and the number of cylinders starts at a V-twin, with no limit after that.
In today's passenger car market, a V-6 often marks an option-up from an I-4, while a V-8 usually lands in a truck, high-end luxury car, or sports car.
 
Longitudinal or Transverse
The rear of the engine is commonly bolted to the transmission, so the question is which direction the engine faces relative to the vehicle.
Longitudinal technically indicates the engine's crankshaft is parallel to the vehicle's direction of travel. A common and often favorite example would be V-8 rear-wheel drive. The drive belts under the hood face forward, and the engine is bolted to a transmission, drive, or propeller shaft coupled to the rear differential.
Transverse means the engine's crankshaft is perpendicular to the direction of travel, with the engine in most cases mounted crossways under the hood. Front-facing left or right varies by application. There are millions of transverse-mounted front-wheel-drive models on the road today, almost all utilizing a transaxle that combines the transmission and differential in one unit.

Front, Mid, or Rear

 
-- Front
The longitudinal and transverse engine examples above are both mounted up front. There are some exceptions, but usually, a transverse engine is used in front-wheel-drive vehicles, while a longitudinal engine is used in rear- or four-wheel-drive vehicles.
The majority of American cars used to use front-engined, longitudinal V-8s and rear-wheel drive. Today, most engines are still up front, but many are transverse I-4s or V-6s with front-wheel drive.
The biggest advantage of the I-4 or V-6 front-wheeler is economy. By mounting the engine transversely and using an all-in-one transaxle, both powertrain space and weight are reduced significantly. A free extra with the package is improved traction due to engine/transaxle weight above the drive wheels, although the forward weight bias typical to most front-wheel-drive vehicles can compromise driving dynamics
The love for front-engine, rear-wheel-drive cars hasn't disappeared. Rear-wheel drive is notoriously ineffective for traction during adverse weather conditions, as compared to front-wheel drive. But having the powertrain weight distributed more evenly from front to rear (engine > trans > drive axle) provides advantages for good-traction recreational driving, on or off the race track.
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-- Rear
By definition, a rear engine has its center of gravity behind the rear axle. So, it's no doubt in the rear.
Yes, our friend the Patent-Motorwagen was a true rear-engine car, along with the Beetle, and the present-day Porsche 911, with its longitudinally mounted six-cylinder "boxer." But you're not gonna see too many rear-engine vehicles on the road.
The majority of the time, a rear-engine layout isn't practical. You will get better traction at the rear wheels due to weight on the drive axle, but all that weight in the rear can make the vehicle prone to oversteer and may require IndyCar talent to balance power and control.
 
-- Mid-engine
In general, mounting an engine as close as possible to the center of the vehicle's wheelbase yields better, more predictable driving dynamics. A mid-engine layout is often considered the ultimate for vehicle stability. This typically means the engine is positioned in front of the rear axle and behind the driver. This centers vehicle mass and promotes more even weight distribution to all four wheels on turns and while braking. The drawback is loss of cabin space when locating the engine closer to center. This often forces a two-seater configuration; the design used mostly in sports car, supercar, and race car applications.
Most have heard the news of Chevrolet's first-ever mid-engine 2020 Corvette C8. But what if someone told you Corvettes have been mid-engine since the C2 in 1963?
More specific terminology used by some defines C2-C7 Corvettes, along with a few other sports cars, as a front mid-engine layout. Front mid-engine stipulates a vehicle with the engine located entirely behind the front axle and in front of the driver. There are more of these on the road than you might have thought, including many common sports cars and luxury vehicles.
The eighth-generation Chevrolet Corvette will be rear mid-engine and should be fun to drive, whatever you want to call it.

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