How Does It Work?
Graeme Morpeth
 
SMOOTH OPERATORS
How many cylinders should an engine have and how should they be arranged?
Images Milton P. Higgins II & Daimler Archives
 
Land, sea and air. This mantra formed the basis of the company that Gottlieb Daimler established in 1886 when he presented to the world the single-cylinder engine – mounted in a four-wheel motor carriage – he had co-developed with designer Wilhelm Maybach.

The engine, with a volume of 28 cubic inches in a vertical “grandfather-clock” configuration (as seen at “A”, top left in the illustration, opposite) produced ten horsepower.

At the time, Daimler recognized the lightweight engine could be used to power boats and airships, as well as automobiles. This idea was symbolized by the three-pointed star used as the Daimler motor company emblem: The points represented the three types of transport.

These single-cylinder engines were not – and still are not – the most comfortable engines to use; they are subject to serious vibration, as anyone who has ever ridden a single-cylinder motorcycle would attest. By 1900, both Daimler and his competitor Karl Benz had moved to multiple-cylinder engines. In fact, both Benz & Co. and Daimler Motor Company produced engines with four cylinders, though with very different characteristics:

The Benz engine was a 540-cubic-inch monster that produced 33 horsepower at 660 rpm; Daimler’s was a more modest 330-cubic-inch affair making 23 horsepower at a heady 800 rpm. Maybach designed the Daimler engine, and with it he established some design and operating principles that are still used in engines today:

Dual side-mounted camshafts operating inlet/exhaust valves

A positive-displacement water pump and a honeycomb radiator

A gear-driven oil pump for camshaft bearings and cylinder walls

Twin carburetors, one for each pair of cylinders

Maybach left the Daimler company in 1907 and in 1909 founded Luftfahrzeug-Motorenbau GmbH (Aircraft Engine Building Company) with his son Karl (renamed Maybach-Motorenbau GmbH in 1912). Thus, his genius and creativity were lost to Daimler. Nevertheless, from that time until now, Daimler and its successor companies have continued to produce engines for air, land and sea.

As internal combustion engine and automobile technology evolved, engine designs using traditional pistons and cylinders have been developed using 3-, 4-, 5-, 6-, 8-, 10- and 12-cylinder configurations.

One might think that evolution left some designs by the wayside, but today, any gearhead can reel off as many as 10 different configurations: an inline 3-cylinder; inline 4-cylinder, 4-cylinder boxer (horizontally opposed); inline 6-cylinder; V-configuration 6-cylinder (V-6); V-8; V-10, as used in the 1994 Sauber-Mercedes C13 racing car; V-12, boxer 12-cylinder as used in the Sauber-Mercedes C291 racing car; and W-12, with cylinders in a W-shaped configuration, as used in the Bentley automobile.

But why such a variety of configurations? To answer this question, a few fundamental principles need to be reviewed. The forces that take place in an engine consist of two types: moments and couples. As defined by Professor Emeritus Milton P. Higgins II, from his forthcoming book, Automotive Milestones, Industrial Press, 2015:

Moment: A turning force that results from the action of a force on a body when the force is applied away from the center of gravity (CG) or rotational center of the body; the moment’s magnitude is the product of the force times its distance from the center of rotation; its effect is to try to rotate the body. An example is tightening a bolt using a wrench.

Couple: A pair of forces acting on a body; the forces are equal in magnitude but opposite in direction. They are also separated by a distance called the moment arm and their combined effect on the body is to create a turning force that tends to rotate the body. As an example, think of turning a steering wheel by pulling down with one hand and pushing up with the hand on the opposite side of the wheel.

These forces cause an engine to experience three types of motion: vertical movement as a result of the reciprocating masses of the pistons, rods and crank weights moving rapidly in their vertical plane; side-to-side motion due to the rotation of the same masses; and lateral motion, end to end as the cylinders fire at different points and different times along the front-to-rear axis of the engine and create unequal forces along the crankshaft, creating a bucking-bronco effect.

Engines also produce audible or discernible vibrations called harmonics, where the first harmonic is equal to the rotational frequency of the crankshaft (rpm) and the higher harmonics are equal to two, three, four, or more times the rotational frequency of the crankshaft. These harmonics can translate into annoying buzzes and rattles at certain engine speeds.

Because the ultimate goal of the engine designer is to create an engine that produces as much power as possible while running as smoothly as possible to maintain the comfort of passengers, minimizing the combined effects of these phenomena is a great challenge.

These effects can best be understood by looking at a single-cylinder engine and examining what happens when it is actually rotating under power, as seen at “B” top right, opposite. The main moving parts (piston, connecting rod and crankshaft) have to accelerate and decelerate twice per crankshaft revolution, and cannot be fully counterbalanced without reducing the power output; thus, the engine vibrates significantly. The magnitude of this vibrating or shaking force is equal to the mass of the piston multiplied by the radius of the crankshaft and multiplied by the square of the crankshaft speed.

Doubling crank speed will quadruple shaking forces. These forces can be seen in the diagram for a single-cylinder engine over the course of two revolutions.

The situation starts to improve when more cylinders are added to the equation because the forces can, to a certain extent, counteract one another. But the layout of the cylinders then becomes critical. A typical 4-cylinder flat-plane engine illustrates the point, seen  at “C” in the illustration. The major forces are balanced in this engine layout, which comprises two mirror-image systems. The clockwise moments affecting the left-hand pair of cylinders are balanced by the counter-clockwise moments affecting the right-hand pair of cylinders.

The “Four-Cylinder Force Diagram” at “D” shows how shaking forces are balanced across two revolutions. However, the secondary forces are not quite in balance, so engine designers seek to minimize or cancel these forces: Balancing shafts rotating at twice the crankshaft speed are the best but most expensive solution, as used by Mercedes-Benz in a number of engines. At ”E”, an M651 diesel engine has these countershafts in the two round devices below the crankshaft.

From 4-cylinder engines, we can now discuss V-8 engines (first used by Mercedes-Benz in the 1939 W165), which had a 254-horsepower 90-cubic-inch front-mounted V-8. This introduces the subject of crankshaft planes. In its simplest form, a V-8 is basically two inline 4-cylinder engines sharing a common flat-plane crankshaft, and suffers the same secondary balance problems as two straight-4s, resulting in excessive vibrations in larger displacement engines.

Most racing V-8s continue to use flat-plane crankshafts because they allow faster throttle response and more efficient exhaust-system designs. In contrast, modern road car V-8 engines use cross-plane crankshafts, as shown in the diagram, in which the crank pins are arranged at 0, 90, 180 and 270 degrees: This balances the secondary moments but introduces primary moments, which fortunately can be eliminated by adding extra balance weights on the crankshaft at 0 and 180 degrees.

The result is an engine that runs more smoothly than a V-6 and is notably less expensive than a V-12. It should be noted that setting the 2-cylinder banks at 90 degrees to one another (the “included angle”) is the best design for a V-8; anything more or less produces those pesky imbalances and causes the engine to run roughly.

Note that we have not covered the V-twin engines – those beloved of the motorcycle industry – because they aren’t used in Mercedes-Benz automobiles. Similarly, we will leave discussion of the 5-cylinder engines for another day: They are no longer used by Mercedes-Benz in sedans (though the OM602 was one of M-B’s finest engines, ranking alongside the OM617 for reliability and longevity). We’ll also skip the V-10, though the 1994 Sauber Mercedes C13 had a 184-cubic-inch V-10 that produced 756 horsepower at 11,600 rpm, because it was only used on that vehicle.

On the other hand, the inline 6-cylinder engine has been a staple Mercedes product since the first 1906 Grand Prix engine, a huge 661-cubic-inch affair that produced a sturdy 106 horsepower at 1,400 rpm: compare this with the 1994 190-cubic-inch M104 engine used in the C36, which produces approximately 285 horsepower at 5,750 rpm.

This configuration of six cylinders illustrated in the diagram has phase angles (the angles at which the crank throws are arranged with respect to the first throw that is always defined as 0 degrees and is usually at the front of the engine) of 0, 240, 120, 120, 240, and 0 degrees, respectively. This gives six evenly spaced firing/power pulses and complete balancing of the shaking forces and moments throughout the rev range as shown in the diagram; it is considered to be one the best for inertia balance and smooth operation.      

The M104 inline-6 was dropped by Mercedes-Benz and replaced by the equally smooth M112 V-6 engine first used in the 1997 C-Class W202 model range, with the cylinder banks set at 60 degrees, apparently because the V-6 offered an easier route to reduced emissions. At the time of this writing, it is rumored that a new inline-6 engine may be introduced in “the near future.”

It might be noted that the W450 Smart car models use 60-cubic-inch 3-cylinder diesel and gasoline engines, which are not quite as smooth as their 6-cylinder big brothers due to their phasing and firing sequences.

A V-12 engine as first used in the 1938 Rekordwagen – a 650-horsepower, 310-cubic-inch beast – has an inherent advantage in terms of smoothness, though only if it has a 60-degree angle included between the banks for the same reason as in the V-6 engines. In modern times, the M120 V-12 engines were first used in 1991 W140 S-Class and 1994 R129 SL-Class automobiles, which gave them spectacular and extremely smooth performance. And just so we know, Mercedes produces V-16 and V-20 diesel engines (M839 and M518s, respectively).

Selecting which engine to employ from the wide range we have discussed depends upon a number of different factors: How expensive will the engine be to manufacture? The most commonly produced engine in the M-B range is a 4-cylinder inline; the least is a V-12. What is the target market for the engine/car combination – a C-Class for the technocrat or an S-Class for the plutocrat? How many model variations are to be introduced into the series? For example, the W205 C-Class has engine options ranging from a 132-cubic-inch 4-cylinder diesel to a 240-cubic-inch turbocharged AMG V-8                  
  
So then, with engines as with most things in life: “You pays your money and you makes your choice.”
 
My thanks to Robert L. Norton, P.E., and Worcester Polytechnic Institute’s Distinguished Professor Emeritus Milton P. Higgins II, who provided definitions and illustrations from his forthcoming book, Automotive Milestones, Industrial Press, 2015.
 
 

CLOCKWISE FROM TOP: (A) Daimler motor carriage with single-cylinder “grandfather-clock” engine; (B) Single-cylinder force diagram; (C) 4-cylinder engine; (D) Force diagram; (E) M651 engine with balancing shafts. Poster of a Mercedes 16/45 PS Knight automobile and stylized Daimler-engined aircraft, 1917. Paris Auto Show, 1898: Gottlieb Daimler (right) and Wilhelm Maybach (behind Daimler, with goatee) in front of a 5-ton truck powered by a 2-cylinder Phoenix engine. Street scene, Daimler Motor Company, Untert¸rkheim, 1908.


 
LEFT COLUMN FROM TOP: Inline and V-configuration 6-cylinder crankshafts. MIDDLE: V-configuration 8-cylinder crankshaft. RIGHT COLUMN FROM TOP:  Examples of flat-plane and cross-plane crankshafts. Flat-plane crankshafts allow for faster throttle response and are often used in racing engines.