The most important mechanical system in any automobile is the engine. Sure, there are plenty of systems that are essential, without which motion is impossible, but the engine is what turned the carriage into the horseless carriage. For most vehicles, the motor is still an internal combustion engine, with air intakes, exhaust manifolds, drive belts, camshafts, and cylinders stuffed full of reciprocating pistons harnessing the fury of exploding dinosaurs to putter to and from the grocery store.
The internal combustion engine is an incredibly complex piece of machinery. The sheer number of parts, the diverse array of supporting systems, and the myriad design choices add up to infinite possibilities. Not every combination is going to work. Fortunately, more than a century of innovation has produced thousands of combinations that do work, almost all of them stemming from two basic configurations that we’re here to talk about today. If you’ve ever wondered about the difference between inline and V-type engines, and why a designer might choose one layout over the other, you’ve come to the right place.
The Original and the Upstart
V-type and inline (or “straight”) engines both trace their origins to the 19th century. While Herr Maybach worked on a 2-cylinder V-type with the cylinders placed at an angle to each other, Mr. Ford built a 4 hp quadricycle using an inline-2 (I2) with parallel cylinders, all before 1900. So it’s a little disingenuous to call V-type engines an “upstart” configuration. However, the layout didn’t see widespread adoption until the 1950’s, and it wasn’t really until the 1990’s that they took over as the predominant engine type on the road. Up until that time, from the 1909 Model T onwards, the industry was dominated by the inline engine layout.
It wasn’t all I4s either, which is a common design today. I6 and even I8 engines were in abundance. The year 1931 featured both the Deusenberg Model J’s 4.2L I8, an advanced model with dual overhead cams and four valves per cylinder making 265 hp, and the Bugatti Type 41 Royale which mustered 300 hp from a gargantuan 12.7L I8 of its own. Though Jeep is better known for 6-cylinder models today, the original Willys-Overland Jeep, created to withstand the abominable conditions of World War II, had an I4, one which was quite literally bulletproof and sparked the entire Jeep brand into existence.
But the straight engine’s dominance wasn’t meant to last. In 1951, Studebaker added a 2.3L 120 hp V8 to their Champion Starlight’s options list, ahead of their 85 hp I6. This powerful (for the time) engine was also the most efficient block on the market, making 28 mpg, which was great for a V8 even by today’s standards. The Chevy Corvette, released in 1953 with the 150 hp “Blue Flame” I6, nearly failed under the stigma of the powertrain’s traditionally mediocre output, despite being more powerful than Studebaker’s V8. The Vette got a 195 hp V8 in 1955, and the rest is history.
Today it’s safe to say the market is split. While the Morgan Three Wheeler sports what looks like a Harley-Davidson V-twin hanging off the nose, and Saab dabbled with V4s back in the day, small V-type engines never really took off for automobiles. Anything with fewer than six cylinders continues to run a straight configuration, and with the proliferation of turbochargers they do well on both fuel and power. For six cylinders or more, though, the configuration is almost exclusively V-type. You simply will not find a modern automobile with a straight-8 engine. Why this disparity? It comes down to the pros and cons of each configuration.
The Inline Engine
Pros
There’s one exception to the general rule about inline engines today. BMW adheres religiously to a tradition of producing an I6 engine. In fact, today’s B58 is regarded as one of the best powerplants on the market. The tradition of inline BMW engines hearkens back to their roots in 1917, when the newly-incorporated amalgamation of businesses produced their first airplane engine. (The BMW logo represents a spinning propeller, as well as the colors of Bavaria.) The inline configuration was essential to the design because it allowed for a narrow, aerodynamic fuselage. It was also as smooth as silk, which is important because early aircraft were fragile and could literally be shaken to pieces by an unbalanced engine.
Therein lies one of the straight engine’s key strengths. With all of the pistons moving in the same plane, the incredible forces and moments of inertia they experience and transmit to the car are naturally balanced out. The I6, in particular, features a perfect harmony. Whenever two pistons are at top dead center, there are two at 120 degrees traveling down and two at 240 degrees traveling up, completely canceling the vibrational forces that would otherwise be transmitted to the car.
The balance of a straight engine also leads to remarkable versatility. One of the most remarkable things about BMW’s B58 I6 is that they use it in everything, and performs as if it were purpose-built just for whichever market segment you find it in. From the M240i sports car, to the 540i luxury sedan, even to the big, beefy X7 xDrive40i–heck, the B58 is in the BMW-based Toyota Supra–the engine delivers exactly the kind of character you’d want from whatever type of car it’s in. It provides nearly instant torque from almost any point in the rev range, making it a responsive and engaging sports car engine. It’s powerful enough to handle huge SUVs, yet buttery smooth enough to be right at home in a luxury sedan, even in hybrids.
Another key feature inherent to the straight engine’s design is its simplicity. With all of the cylinders oriented in the same direction, the engine needs only one cylinder head, one head gasket, and one valvetrain. Fewer simpler parts means less cost, not just to customers, but for development as well. In fact, part of a modern resurgence of I6 engines can be attributed to the simplicity of developing an I6 based on a well-established I4 design, compared to chopping two cylinders off a V8 (like GM’s 4.3L V6) or designing a totally unique V6.
Fewer parts means also fewer points of failure, and inlines are renowned for reliability. There’s the aforementioned Jeep I4 of course, but GM, Chrysler, and Ford used inline engines in cars and trucks for upwards of 30 years. If it ain’t broke, why fix it?
Shoot, even if it is broke, that upright packaging leaves tons of room in engine bays for getting in there and fixing them up at home. You might seek out an inline engine for this reason if you’re a DIY mechanic. Packaging, though, is also a con for inline engines.
Cons
The problem is, as the cylinders add up, the engine bay gets long. Consider pre-war I8 designs to see some obvious examples. That’s not great if your objective is to build an econobox with an aerodynamic front end. What if your objective is to build a lightweight front wheel drive platform? Now that long engine needs to be turned sideways, with the cylinders in a row that goes from side to side, and has to somehow squeeze between the wheel wells. Volkswagen and Volvo have used 5-cylinder engines, but typically you see either a I4 or a V-type under the hood of a front-wheel drive car.
As design requirements changed to demand shorter noses and transverse positions, and V-type engine reliability improved, the straight engine began to decline. The I8s disappeared in the 1950’s, and I6’s largely vanished in the 90’s, at which time the world of big engines turned almost entirely to V-types.
The V-Type Engine
Pros
Though the V-type wasn’t invented to answer the straight engine’s packaging problem, it’s absolutely the reason why V-types took over. Square engine bay geometry seems tailor-made for a square engine block, which is exactly what happens when two I3s are joined at the crank and offset at an angle to one another, instead of lined up front to back, to create a V6 engine. More cylinders means more benefit. Spaces that could only accommodate an I4 could easily accommodate V6 and even V8 engines. This allows more powerful engines to fig under the hood of the same body design, or enables designers to achieve the same power from a smaller, more efficient body design.
The greatest benefit is realized for transverse installation in FWD platforms. With less weight, higher efficiency, and greater control than RWD in slippery conditions, it’s no wonder that FWD took over once V-types were reliable enough to produce on a large scale.
Over the years, manufacturers have experimented with an incredible array of configurations for V-types. The angle of the V is a signature parameter. Volkswagen has used an angle as small as 10 degrees in the VR6 engine, enabling the use of a single cylinder head on a V-type engine to try having the best of both worlds. Ferrari and Porsche have gone to the opposite extreme, producing a 180 degree V. How this is different from a boxer is a topic for another day.
The design also allows for creativity in the design of the crankshaft. American V8s classically choose a cross-plane design that helps produce a ragged rumble in combination with a somewhat awkward timing scheme. Iconic European Vs, and the new Corvette Z06, use a flat plane design that enables higher revs and a hair-raising howl. But that might be where the benefits end.
Cons
V-type engines are inherently more expensive to produce and to maintain. The offset banks of cylinders produce forces that don’t necessarily cancel out. When the banks have odd numbers of cylinders, balance shafts are often required to keep the engine from shaking itself apart. The only truly balanced V-types are V12s, which are essentially formed from two I6s. Thus, V-types demand a higher degree of testing and iteration, and generally offer more opportunities for something to go wrong due to the need for two valve trains, two cylinder heads, etc.
This was a bigger problem 50-60 years ago than it is today, but it’s still worthy of note that a V-type is generally more expensive to buy and own than a straight engine. The reason the V4 never took off for automobiles is that it doesn’t make practical sense. The I4 that it might replace is already compact enough for any engine bay, so why add the complexity and double the number of parts involved? Even the V6 is losing some ground in today’s “universal platform” industry. As mentioned before, manufacturers such as Mazda are capitalizing on good I4 designs by extrapolating them to I6s instead of developing standalone V-types.
But in the End…
These are really all just technicalities for most drivers. One of the biggest things you’d notice as the owner of a straight engine or a V-type is the sound it makes. If an automaker wants to produce big power, make something really efficient, have a broad torque band, or reach a super-high redline, they’ll find a way to do it using whatever engine design fits under the hood.
Basically, the difference between inline and V-type engines is that a V-type is two smaller inline engines joined at the crankshaft and offset at an angle to one another. Yet the story behind how the two most common automotive engines came about, where they were used, and how they’ve managed to land a near-even split of market share runs surprisingly deep, a fitting history for a device that runs on something as exotic as exploding dinosaurs.