Compression Ratio, Cylinder Size, And Engine Speed

Detonation and engine-design features related to modern motorcycle engines.

BMW R 1800 boxer twin engine
Air- and oil-cooling, four valves per cylinder, pushrod actuation, and 9.6:1 compression: Peak torque—116 pound-feet—for the new BMW R 1800 boxer twin arrives at 3,000 rpm. A 107.1mm bore (4.22 inches) and 100.0mm stroke (3.94 inches) deliver 1,802cc or 110ci.BMW

In my recent story about BMW’s new R 1800 “big boxer,” I applied the adjective “substantial” to its 9.6:1 compression ratio but did not add any detail. A reader rightly wondered how I could use the term “substantial” when Ducatis with equal or larger cylinder bores operate on much higher compression ratios—more than 12:1.

Yep, there’s more to this than just compression ratio. Every type of spark-ignition engine fueled by gasoline is given the highest compression ratio it can safely use on the intended fuel. The reason is that the higher compression can safely be raised, the greater the torque the engine can give and the more economically it can run. Higher compression puts more of the fuel’s energy to work on the piston as combustion pressure, wasting less of that energy as heat and pressure released into the exhaust system.

Detonation sets the limit to compression. When heated to high temperature for long enough, a mixture of gasoline and air becomes progressively less stable. Intense thermal collisions knock some of the most vulnerable hydrogen atoms off fuel molecules, and those hydrogens may unite with oxygen—present in the mixture as O2 molecules—to form the highly reactive radical OH-. If the population of this OH- radical grows large enough by sufficient time exposure to high temperature, bits of the last mixture to burn may auto-ignite and then burn at the local speed of sound before the normal flame front can reach and consume them. This is detonation, also called “engine knock” because when it occurs we can easily hear the sharp metallic sound of its sonic shock wave hitting combustion chamber and piston surfaces.

In normal combustion, a spark-initiated flame front is shredded and mixed by turbulence in the unburned mixture, creating so great an area of flame that the mixture burns rapidly at an apparent speed of 50 to 200 feet per second. Normal combustion reaches the cylinder wall, consuming the mixture before a dangerous OH- population can form. Normal combustion makes no sound because it is a relatively slow progressive process. The familiar sound made by piston engines is the sudden expansion to atmosphere of the low pressure remaining in the cylinders when their exhaust valves open.

Why do so many writers refer to piston engine combustion as “an explosion”? Search me. The reaction front created when an actual explosive such as TNT, PETN, or TATB are ignited moves at 20,000 to 30,000 feet per second, or several hundred times faster than combustion in engine cylinders.

1299 Panigale R Final Edition
Boom-boom room: Ducati spec’d 13.0:1 compression for the street-going 1299 Panigale R Final Edition. Bore and stroke was 116.0mm x 60.8mm for a displacement of 1,285cc. The factory claimed 209 hp at 11,000 rpm and 104.7 pound-feet of torque at 9,000 rpm.Ducati

Okay, the two major variables tending to foster detonation are temperature and time. When the temperature of part of the last mixture to burn rises high enough, OH- radicals begin to form. The longer those bits of mixture are held at high temperature, the higher the OH- population rises, possibly enough to result in detonation. Therefore, engine designers work to limit the temperature rise of the mixture. A common example is the use of a charge air cooler (“intercooler”) to remove the heat of compression added to the mixture by a turbocharger. The hotter the mixture entering engine cylinders, the more likely detonation becomes.

As engine rpm dropped on the steep climb, there was more and more time for the formation of OH- radicals in the hot cylinders, and those radicals did their noisy destructive work.

Next is the time factor. As the spark occurs and the flame front expands, consuming the unburned charge ahead of it, that unburned charge is itself being compressed and heated. To keep that process from going too far, engineers work to speed up combustion so it can win the race against the formation of OH- radicals. The time factor also explains why detonation can result for too-advanced ignition timing; it makes combustion take more time. Designers use high intake velocity and piston-to-head squish to make the charge turbulent, thereby speeding combustion.

Years ago, my parents’ family car was a 1951 Kaiser with a low-compression Continental Gold Seal flathead six. If we climbed a steep hill that pulled engine rpm down to “lugging speed,” I could hear the rattle of detonation, and the parent at the wheel would shift down and the detonation would stop. The reason? Time. As engine rpm dropped on the steep climb, there was more and more time for the formation of OH- radicals in the hot cylinders, and those radicals did their noisy destructive work. Shifting down increased engine rpm and shortened the time exposure of unburned mixture to high temperature, enough to cause knock to cease.

No one is going to ride a Ducati sportbike with giant cylinders on full throttle at 1,300 rpm (Ducati got up to 112mm in its World Superbike V-twins before switching to the V4 R with four 81mm cylinders). There are two good reasons for this: First, in general, the bigger the cylinder bore, the longer combustion takes, so lugging it down to 1,300 rpm is just asking for detonation. Second, with its longer valve timings aimed at making higher-rpm power, that Ducati is weak as a kitten at 1,300 rpm. To make power at higher revs, it’s necessary to leave the intake valves open for quite a while after bottom center to let the high-speed intake flow keep coasting into the cylinders. But when we try to ride with such late intake closing at 1,300 rpm, the intake flow is moving too slowly to keep coasting into the cylinders. So the rising piston stops the flow and reverses it, pushing mixture back out of the cylinder. What is trapped in the cylinder when the intake valves finally close is not a full charge, making torque weak.

BMW’s R 1800 is designed with very short valve timings to give it the low-speed “grunt” that cruiser riders want, and the result is a powerband usable from 1,300 to 4,750 rpm. Its intake valves close very soon after bottom dead center (BDC), allowing it to trap a full cylinder’s worth of charge when running at low revs; as the engine revs up, its cylinder filling falls above 3,000 revs. That full cylinder charge is where the 1800’s low-rpm grunt comes from. But with its combination of big cylinders (107.1mm), low rpm, and excellent low-speed cylinder-filling, that engine would knock itself to death by detonation if it were given the 12:1 compression that works fine in the big Ducati twins at their much-higher normal rev range (and reduced time for the formation of OH- radicals).

When I called the R 1800’s 9.6 compression “substantial,” I meant in relation to that engine’s special circumstances: large bore and operation at rpm that for other engines would be considered lugging (and abuse for a Ducati Superbike). Compression has been pushed as far as it’s safe to go in that design.

Further information comes from BMW’s fuel callout: 95–98 RON (Research Octane Number). A special fuel callout indicates some concern that detonation might occur on less-than-premium fuel. And that, in turn, indicates that the engine was given as much compression as would be safe. So 12:1 is close to the limit for a Panigale V-twin, and 9.6:1 is close to the limit for the R 1800.