Engine Bore vs. Stroke

Bore refers to the diameter of a single cylinder and defines the projected surface area of the face of the piston. As bore increases, so does the volume of the cylinder and therefore the amount of air that the cylinder can hold at any given time.

Stroke refers to the length the piston travels from TDC (top dead center) to BDC (bottom dead center). As stroke length increases, volume of the cylinder also increases in reference to the amount of air that can be drawn/forced during the intake stroke.

The ratio of bore diameter to stroke length is known as the bore/stroke ratio. A “square” engine will have a bore/stroke ratio of 1, meaning the bore diameter is equal to the stroke length. An “under square” engine has a bore/stroke ratio less than 1, meaning the stroke length is larger than the bore diameter. And an “over square” engine has a bore/stroke ratio greater than 1, meaning the bore is larger than the stroke.

Keeping in mind that there are exceptions, most diesel engines are under square, meaning they've been designed with a stroke that is longer than the engine bore diameter. This is because a relatively long stroke promotes torque production, especially at low engine speeds. As stroke length increases, the lever arm created between the connecting rod and crankshaft also increases (called the crank throw). The additional leverage creates a greater moment (engineering equivalent of torque) on the crankshaft and this translates into more torque at the flywheel.

The downside of a long stroke is increased piston velocities at any given engine speed. The best way to visualize this is to compare two engines. Imagine engine A has a 3.5” stroke, while engine B has a 4.5” stroke. At 2,000 rpm, the crankshafts are rotating at exactly the same speed. However, engine B's piston must travel 4.5 inches in the same amount of time that engine A's has to travel 3.5”. Therefore, engine B's piston will be traveling faster as it approaches BDC. The increased piston velocities limit the maximum engine speed that can be safely achieved repeatedly without compromising longevity.

Engine Balance - I6 vs. V8

The internal balance of an engine is divided into two categories - primary balance and secondary balance. Primary balance refers to the balance of the forces created by the piston/rod mass as it changes direction from up to down (and visa versa). An engine that has primary balance will therefore have equal changes in inertia; as one piston changes direction from upwards to downwards, an opposing piston is changing direction from downwards to upwards on the same relative path, but in the opposite direction. The forces then theoretically cancel out if the component masses are identical from cylinder to cylinder. For most engines, this means that manufacturing tolerances and control of component mass are the main factors of primary balance.

Secondary balance is more complex and is dependent on the configuration of the engine. To fully grasp the concept of secondary balance in a rotating engine, it's necessary to first understand the relationship between piston speed and crankshaft angle.

1) The velocity of a piston is fastest for the 180 degrees of crankshaft rotation that it is traveling upwards or downwards in the cylinder. 2) The velocity of a piston is slowest for the 180 degrees of crankshaft rotation where the piston changes direction. During this time, the piston must momentarily stop in order to change direction, then it proceeds to accelerate in the opposite direction.

In a V-8 engine, the combined forces of the pistons on a fast approach are not canceled out by the forces of other pistons changing direction (slow approach). This is inherent to the fact that a V-8 has crank throws 90 degrees apart (with rare exceptions) and 8 pistons. Therefore, there is a positive net force creating a vibration in the vertical plane whose frequency is equal to twice the engine speed (a 90 degree crank throw multiplied by 8 cylinders is 720 degrees, or twice that of one complete engine rotation). Counterweights in the crankshaft (balance shafts) are used to minimize the effects of secondary imbalance.

For the purpose of comparison, an inline 6 engine can be thought of as two 3 cylinder engines. Both configurations utilize crank throws 120 degrees apart. In this arrangement, the combined inertial force of two pistons moving downwards is equal to, and therefore cancels out the inertial force of a third piston traveling upwards. The inertial forces are relative to the position and therefore velocity of each piston, but the net inertial force of the three pistons will always be zero. Therefore, the system inherent experiences secondary balance.

Inline 6, V-8 Advantages & Disadvantages

It is often thought that the architecture of an inline 6 engine favors torque production over a V engine design, when in fact the arrangement of cylinders has little, if anything to do with performance characteristics. However, most inline engines employ longer strokes to compensate for smaller bore diameters since the engines total length (longitudinally) is relatively long. It also often thought that inline engines produce torque at lower engine speeds than V engines, when in fact low engine speed torque is inherent of the diesel cycle and also a benefit of a long piston stroke. The combustion process in a diesel engine yields a long duration "push" on a piston, as opposed to the explosive "punch" a gasoline engine enacts on a piston. The engine speed at which peak torque is produced depends on much more than the configuration of the engine.

Inline 6 engines are used overwhelmingly by medium and heavy duty truck manufacturers for a variety of reasons. First, the configuration frequently allows for plenty of access to perform overhauls and repairs without requiring removal of the engine from the chassis. Secondly, the I-6 is naturally balanced, which produces smooth power generation and minimal internal vibration. Finally, to the benefit of engine manufacturers, an I-6 (being naturally balanced) can be scaled up or down in size with relative ease, reducing costs in design and manufacturing processes.

The I-6 does have a inherent downfalls, the most profound of which is typically low operating speeds. If you've ever been around a tractor-trailer, you may have noticed that the red line is typically in the 1,800 – 2,000 rpm range. The two factors that limit engine speed in inline engines are the long crankshaft and piston stroke. A long crankshaft is subjected to considerably larger torsional deflection than a shorter shaft, and this deflection becomes greater as engine speed increases. Additionally, long strokes translate into faster piston speeds, which can cause excessive wear and reliability concerns at higher engine speeds.

I-6 Diesel Advantages & Disadvantages

Advantages Disadvantages Longer stroke promotes torque output at low engine speeds. Lower operating speeds. Ease of maintenance and/or repair in most applications. Larger torsional stresses applied to the crankshaft. Naturally balanced, contributes to smooth power generation. Maximum bore size limited by engine length (as the bore size increases, so does the length of the engine).

A "V" engine configuration also exhibits inherent advantages and disadvantages. A V engine is significantly shorter than an inline 6 engine of comparable bore and stroke dimensions. As such, V-8 engines tend to fit in space limited applications. Earth moving dump trucks, for example, typically employ large displacement V-12 and greater diesel engines - to meet the power and torque requirements of such vehicles would require a tremendously long inline engine. Pickups and passenger cars are also situations where V engines are favored, the Cummins powered Ram topping the list of exceptions.

The shortened nature of a V-8 engine always requires a shorter crankshaft, which yields favorable results with respect to wear caused by high torsional forces and engine speeds. Higher operating speeds translate into a broader range of usable power and a V-8 is therefore inherently versatile.

V-8 Diesel Advantages & Disadvantages

Advantages Disadvantages Shorter dimensions for a given displacement Natural secondary imbalance Lower torsional forces on crankshaft due to shorter crankshaft length Overall engine width may increase difficulty of maintenance and/or repair due to limited access Higher operating speeds, broader range of usable power

There are obvious exceptions to the principles discussed herein, which warrants the fact that the performance characteristics of an engine design depends on a number of important factors. However, engine configuration alone is only one variable and does not necessarily dictate specific performance characteristics.