Otto Explains — Car Technology
Different Engine Designs Explained: Inline, V, Boxer and More
Short Answer
The main difference is how the cylinders are arranged. An inline engine has them in one row, a V engine splits them into two angled banks, and a boxer lays them flat on opposite sides of the crankshaft. The arrangement decides the engine’s length, width and height, how smoothly it runs, and how easily it fits a particular car. A rotary engine is different again: it has no pistons at all.
What is an inline engine?
In an inline, or straight, engine all the cylinders sit in a single row along one crankshaft. It is the simplest arrangement, and most four-cylinder car engines are built this way. Inline engines are narrow but tall, and they grow longer with every cylinder: a straight-six is narrower than a V6 or V8 but longer than a four. An inline-four has a natural imbalance that causes a slight up-and-down vibration at twice the speed of the crankshaft, so larger ones often use balance shafts to calm it.
Why is the straight-six known for smoothness?
With six cylinders in a row, the forces from the moving parts cancel each other out. BMW, which has built straight-six car engines for more than seven decades, says this balance removes the need for a balance shaft, which saves weight and helps the engine run smoothly. The price is length: a straight-six is a long engine, which is one reason many manufacturers replaced it with the shorter V6.
What is a V engine, and why is it so common in larger engines?
A V engine splits its cylinders into two banks set at an angle to each other, sharing one crankshaft, so seen from the front the engine forms a V. Because the cylinders are in two shorter rows, a V engine is shorter than an inline engine with the same number of cylinders, though wider. That compact length is why V6, V8 and V12 are the most common layouts for car engines with 6, 8 or 12 cylinders. Some designs blur the line: Volkswagen’s VR6 uses an angle of only 15 degrees between its banks, against the 50 to 90 degrees typical of a V, so both banks share one cylinder head and a six-cylinder engine fits crosswise in a compact car.
What is a boxer engine?
In a boxer, or flat, engine the cylinders lie horizontally on opposite sides of the crankshaft. Each piston has its own crankpin, and opposite pistons move towards and away from each other, which is where the name comes from: Subaru describes the motion as punch and counterpunch. Because the engine is flat, it can be mounted low, lowering the car’s centre of gravity, and the opposing movement can reduce vibration. The trade-off is width: a boxer is wide and flat, so parts on either side can be harder to reach, which Subaru notes can make it more complex to package and service. Subaru uses boxer engines in all its combustion-engine models.
What about rotary engines?
A rotary, or Wankel, engine has no pistons moving up and down. Instead, a rotor shaped like a rounded triangle turns inside a specially shaped housing. The design is compact, smooth and light, but it tends to use relatively more oil and to burn its fuel more slowly and less completely than a piston engine. Mazda is the best-known user: it mass-produced more than 2 million rotary engines between the Cosmo of 1967 and the end of RX-8 production in 2012. Since 2023 it has used a compact 830cc single-rotor engine in the MX-30 e-Skyactiv R-EV purely as a generator, with no mechanical connection to the wheels.
Does the engine layout change how a car should be looked after?
Not the basic rules. Whatever the layout, the manufacturer’s service schedule and specifications apply. Layout mainly affects how easy parts are to reach and how a fault shows itself. On a wide boxer engine, for example, parts on either side can be harder to reach. And because a well-balanced engine runs smoothly, a new vibration is worth noticing whatever its design: it is more often a sign of a fault such as a misfire or worn engine mounts than of the layout itself, as explained in why a car vibrates at idle. AutoTronix Garage diagnoses and repairs engines of every layout, and AutoTronix Express handles scheduled maintenance.
Common Questions
- Is a V6 better than a straight-six? Neither is better in every way. A straight-six is naturally better balanced and narrower, while a V6 is shorter and fits more easily into many engine bays. Manufacturers choose according to the car they are building.
- Why do Subaru cars use boxer engines? Subaru points to two benefits: the flat engine sits low, lowering the car’s centre of gravity, and its opposing pistons can reduce vibration. It uses boxer engines in all its combustion-engine models.
- What does VR6 mean? It is Volkswagen’s name for a six-cylinder engine that combines elements of an inline engine and a V engine. Its two banks sit only 15 degrees apart and share one cylinder head.
- Are rotary engines still made? Yes, by Mazda, though no longer to drive the wheels. Since 2023 a single-rotor engine has served as a generator in the MX-30 e-Skyactiv R-EV.