A Swan Song for the Inline-four Sportbike?

Admin · December 30, 2022

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Suzuki’s GSX-R1000 is being discontinued in the Japanese and European markets, and in the future other sportbikes may be produced for trackday use only. What is driving such changes?

Euro 5 emission standards cuts the limit for unburned hydrocarbons (UHC) in the exhaust from 0.17 gram per kilometer to 0.10, carbon monoxide from 1.14 gm/km to 1.0, and nitrogen oxides (NOx) from 0.09 gm/km to 0.06.

Unburned hydrocarbons make their way into a motorcycle engine’s exhaust stream in a variety of ways:

The compression stroke pushes small volumes of fuel-air mixture into piston-ring  and head-gasket crevice volumes, from which they later emerge to join the exhaust stream as UHC.
Some fuel is absorbed into cylinder-wall oil film or combustion-chamber deposits and later evaporates from them.
Many propose that combustion is quenched by close proximity to cooler metal surfaces.
Fresh air-fuel mixture short-circuiting from the intake to the exhaust during the valve-overlap period.
Poor oil control by piston rings and valve-guide seals.
Incomplete combustion in slower-burning rich or lean mixture zones.
Actual misfire.
Evaporation of fuel from the fuel tank.

Of the above, 1 and 2 are most important, while 5, 6, and 7, and 8 are either already minimized by existing technologies or are matters of engine maintenance. No. 3 remains to be proven, while 4 depends upon the length of the valve-overlap period (high-power racing or sport engines tend to use longer overlap periods).

Piston-Ring and Head-Gasket Crevice Volumes

To find the total length sealed by piston rings or head gasket, we take the circumference of one cylinder (the distance around it—𝜋 [3.1416] times cylinder bore) multiplied times the number of cylinders.

To get a specific measure in terms of sealed length per unit of displacement, we can then divide this number by the engine’s displacement in liters.

The latest GSX-R1000 has 76mm cylinder bores. Since 𝜋 is roughly 3.14, the sealed length for one cylinder is 76 x 3.14 = 238.8mm, and the total for four cylinders is 4 x 238.8 = 955mm.

Because the Suzuki’s engine displacement is 999.8cc, our sealed length per liter for this engine is about 955mm, or about 37 inches.

Now, let’s take a look at an engine designed to meet Euro 5 from the same company: Suzuki’s recently released parallel-twin 776. The bore and stroke are 84 x 70mm, with the circumference of one cylinder 84 x 𝜋 = 264mm. For the whole engine the figure is twice this, or 528mm. To convert this number into seal length per liter, we divide by 0.776 (the engine’s displacement as a fraction of a liter) and come up with 680mm.

This number is just 71 percent as much as the GSX-R1000′s seal length per liter, which gives the more modern engine’s developers a head start in controlling UHC emissions. The less total length of piston-ring and head-gasket crevice volume, the less UHC can hide from combustion there, only to emerge later in the cycle to flow out with the exhaust.

The really strong variable here is the number of cylinders. Changing from four cylinders to two reduces the sealed length a lot. Changing from the GSX-R1000′s big-bore/short-stroke design to the 776′s less extreme bore/stroke does reduce sealed length as well, but by a lesser amount.

Reducing the Number of Cylinders

To further understand how powerful reducing the number of cylinders is in reducing sealed length per liter, let’s imagine converting the latest GSX-R1000 four into a twin having the same bore/stroke ratio. The present engine has a bore/stroke ratio of 76/55.1 = 1.38. A 1,000cc twin with the same ratio would have bore and stroke of 95.8 x 69mm, giving us a ring- and gasket-seal length of 95.8 x 𝜋 x (number of cylinders) = 602mm, a substantial reduction from the four-cylinder’s 955mm.

But the result—a twin with a much longer stroke—would never have been reliable at the rpm required to give GSX-R1000 power.

Meeting the Euro 5 UHC Limit

To increase the GSX-R1000′s horsepower in recent years and enable it to reach higher revs without subjecting its pistons to damaging levels of piston acceleration, Suzuki had to take the opposite approach. The engine’s stroke was shortened and its bore enlarged more than once. Twenty years ago the bike’s bore and stroke were 73 x 59mm, and then 10 years ago that was changed to 74.5 x 57.3mm. BMW, in seeking to make its four-cylinder S 1000 RR engine king of the hill, went even further—to 80 x49.2mm. In terms of sealed length per liter, the BMW is at 80 x 𝜋 x (number of cylinders) = 1,005mm per liter of displacement.

Why Not Just Do the R&D To Make GSX-R1000 Meet Euro 5?

How does a manufacturer decide whether or not to invest in the R&D necessary to develop a given model to meet Euro 5 emissions? This is a simple cost/benefit calculation.

If big sportbikes were still selling in large numbers, income from their continued sale might pay the R&D bill. But the present depressed sales of powerful sportbikes make it very unlikely that R&D dollars spent to make GSX-R1000 meet Euro 5 would earn a profit.

Better to discontinue the model and invest the R&D into new designs that have better sales prospects. Those new designs can be made to meet Euro 5 more easily—and at lower cost. Designs such as those with fewer cylinders, smaller bores, and longer strokes.

Euro 5 and Valve Overlap

Loss of fresh air-fuel mixture during valve overlap has been another major contributor to UHC emissions in high-performance engines. Valve overlap has been useful in boosting performance, providing a “window” through which a low-pressure wave in the exhaust pipe, returning to the cylinder during this period, can suck out exhaust gas remaining above the piston when it is close to TDC, and can then pull into the cylinder fresh charge through the just-opening intake valves.

In effect, long overlap can allow tuned exhaust-wave action to start the intake stroke even before the piston has started to descend. The downside: The longer both intake and exhaust valves are open, the greater the time during which fresh charge entering the cylinder from the intake valves may be lost out the just-closing exhaust valves. Shorter overlap means fewer emissions.

Other Ways To Achieve High Performance

If all of this sounds like the heavy hand of Big Brother coming down hard on our two-wheeled fun, let me suggest that new ways to deliver useful performance are resulting in at least as much fun as before, with an increased sense of control and security. Compelled to find other ways to boost performance caused by shorter valve overlap, engineers are now making up for the loss by lifting the intake valves faster and higher. The resulting engines have flatter, wider torque curves than the long-overlap designs. Another plus: Such engines are easier to ride well.

The raw power of the previous era’s sportbike engines came at a cost in weight: Those literbikes were truly power stations on wheels that frankly frightened many of their riders. The manner in which those engines made their power—mostly up high in the rpm range—made them more demanding to ride, requiring constant attention to the tachometer to keep the engine singing in its best range.

Lower weight and wider power make more recently designed bikes easier to ride well, with less focus on tach watching. The chassis and suspension further benefit from the latest in those technologies, returning enhanced responsiveness. Electronic rider aids have now matured so their effects are less intrusive. The bottom line is that it has become easier to get the performance we want from this new kind of bike because we can safely use more of what is there.

I am with those who loved the high song of four-cylinder engines at peak revs. I also celebrate the ability of our industry to deliver what we want in performance while meeting exhaust emissions standards made necessary by the present world population of eight billion persons.

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