When most of us began to learn about internal combustion engines, our first lesson was usually on the four-stroke cycle, or Otto cycle (named after its inventor, Nikolaus Otto). Intake, compression, power and exhaust were drilled into our heads. It's an oversimplification of a complex series of events that converts chemical to mechanical energy, but at least it's easy to remember. When these steps are applied to a single-cylinder engine, there's not much more we need to think about--after all, there's only one ignition event to be concerned with. But things change dramatically when you think about a multi-cylinder engine. In what order should the cylinders fire? Does it make any difference?
Once an engine incorporates more than a single cylinder, the ignition events get more complicated. Four-stroke engines have to be timed with the arcing of multiple spark plugs. This adds a great deal of complexity. What was simple with one cylinder seems overwhelming on a V-16. And so engineers quickly recognized that a defined order of firing events would be required.
At first glance, it may appear (and I am certain that the early engineers believed the same) that an engine could just fire in numerical order. One, 2, 3, 4 seems logical enough, but there was more going on than meets the eye.
The sequence in which an engine's cylinders are made to fire is chosen based on several criteria. First, you want to improve distribution of the incoming fuel and air charge in the manifold to the bores, and to aid in the evacuation of the spent gases at the end of the power (expansion) stroke. At the same time, the firing order is designed to suppress objectionable and negative torsional vibrations in the crankshaft. The engineers then need to consider all of the consequences of a given firing event order.
It's often beneficial to have successive cylinders fire as far apart as possible. This allows a recovery of charge in the intake manifold and promotes a higher level of volumetric efficiency. It also minimizes the interference between adjacent or nearby cylinders that may have overlapping induction periods. The standard rule is to arrange the firing order so that the cylinders on opposite ends of the manifold or from alternating banks on a V-shaped engine draw alternately. This becomes increasingly difficult as the number of cylinders is reduced.
Similarly, on the exhaust side of the engine, if the discharge periods for close-together cylinders overlap, exhaust-gas back pressure may prevent the spent combustion gases from leaving the bore. Separating or distancing successive cylinders which are in blow-down mode (exhausting) is therefore extremely important--perhaps even more important than the effect of the firing order on the induction or cylinder filling.
When the flame expands in the bore and works against the piston, power impulses have a tendency to twist the crankshaft. If the natural torsional oscillations of the crankshaft happen to coincide with these evoked impulse frequencies, vibration can occur. So it's advantageous to apply successive power pulses to alternate ends of the crankshaft. With some crankshafts, successive firing of adjacent cylinders prevents relative unwinding, thereby suppressing critical torsional vibrations and harmonics.
The arrangement of the crankshaft throws in relation to the grouping of the engine cylinders and their number is determined by two often-conflicting considerations: acceptable engine balance and equal firing intervals.
The placement of the crank throws is also determined by the requirements for even firing intervals of the cylinders and for spacing the successive power impulses as far apart as possible along the crankshaft, so as to reduce the twisting effect. For any four-stroke engine, the firing intervals must, if they are to be equal, be 720 degrees divided by the number of cylinders.
For inline four-cylinder engines, the first and fourth crank throws are therefore indexed on one side of the crankshaft, and the second and third throws on the other side. The firing order of these engines, numbering from the front, may then be 1-3-4-2 or 1-2-4-3 at 180-degree intervals. In the case of a straight-six engine, the throws are spaced in pairs with an angle of 120 degrees between them. Thus, the first and sixth crank throw are paired, as are the second and fifth, and the third and fourth. The firing order may then be such that no two adjacent cylinders fire in succession. It will either be 1-5-3-6-2-4 or 1-4-2-6-3-5 at 120-degree intervals.
For V-shaped and horizontally opposed engines (such as in a Corvair), the firing order follows a sequence similar to that used in inline engines. The crank throws are arranged so that the firing alternates between the cylinder banks as it proceeds along the crankshaft.
The V-6 engine is a unique design that requires special consideration. When Lancia pioneered the production V-6 engine in 1950, they inclined the two banks of cylinders at 60 degrees to each other, which, together with a six-throw crankshaft, created equally spaced firing intervals of 120 degrees. However, this was not generally true of other early V-6 engines. Their designers adopted an included angle of 90 degrees between the cylinder banks and used a shorter, three-throw crankshaft with three pairs of connecting rods, each sharing common, double-length crankpins. This arrangement resulted in firing intervals of 90 and 150 degrees alternatively, instead of even 120-degree intervals. Although these engines were more economical than a V-8, they were not particularly smooth-running.
To overcome the problem of uneven firing intervals with a 90-degree V-6, Buick engineers retained the three-throw crankshaft, but ingeniously replaced the common, double-length crankpins with adjacent single crankpins that were staggered 15 degrees in opposite directions. This produced a so-called split-pin crankshaft. The reason for the 90-degree cylinder spacing on early V-6 engines, especially as designed by Detroit, was that the 90-degree V-8 was the mainstay of their engine production, and this allowed much of the machinery and tooling to be used, thus providing a huge cost savings. In contrast, the European manufacturers that produced a 90-degree V-6 did so for the opposite logic--they wanted the tooling in place so they could move up to building V-8 engines in the future.
Some designers of modern even-firing V-6s have reverted to a 60-degree included angle between the cylinder banks, but have retained a three-throw crankshaft supported in four main bearings. The required angular spacing for the crankpins of each throw is then obtained by using what is called a "flying arm." This can be likened to a crank web that does not connect with a main journal and which may also be used for balancing purposes.
When General Motors introduced its 1980 front-wheel-drive X-cars, it included a new V-6 engine in the lineup. The 2.8-liter (173-cu.in.) V-6 was completely new and was designed specifically for the transverse mounting. Produced at the Chevrolet engine plant in Tonawanda, New York, it was a unique V-6 because it featured a 60-degree design. The Tonawanda workers were very proud of the new design and had bumper stickers and T-shirts announcing the 60-degree V-6 to the public.
This new engine needed to fit transversely under the hood of a small car, so a tight 60-degree spacing between its cylinder banks was required. Also, the first-generation odd-firing Buick V-6 that appeared in the early 1960s (which was reintroduced for 1975) was very reliable and economical, but extremely rough-running due to the variation in firing impulses. Not only did the public reject the engine, but it caused a good deal of collateral damage to GM's reputation for its lack of refinement. The corporation did not want the same rejection to impede its brand-new series of cars.
Thus, the Chevrolet 60-degree V-6 came to market with a unique firing order, 1-2-3-4-5-6. Being a V-shape, the cylinder numbering meant alternate banks were firing, but in a conga-line arrangement. Due to this, the intake manifold was designed to provide efficient cylinder fill while having a straight firing order. The results were stunning, and the little 173-cu.in. V-6 had the slow idle speed and smoothness of a V-8 that most potential buyers were used to. The only obstacle was that it used no gaskets and pioneered the employment of RTV sealant--it was known for oil leaks.
Though the crankshaft design has a major impact on the firing order, the camshaft also comes into play. By modifying the placement of the lobes on the camshaft, the firing order can be manipulated within the confines of the crankshaft throws and pins. Two very common engines that offer two different firing orders are the 302 and 351 small-block Ford V-8 configurations. When modified for extra power, the firing order of two cylinders is switched. This helps smooth the pressure pulses and improve engine breathing and the discharge of spent exhaust gases.
As with most aspects of the internal combustion engine, there is more going on to ignition firing than meets the eye. How the firing order is set can impact an engine's smoothness, longevity, customer appeal, power and economy: A tall order for a benign series of numbers.
This article originally appeared in the March, 2012 issue of Hemmings Classic Car.
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