A Swan Tune for the Inline-four Sportbike?

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>.

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

Suzuki’s GSX-R1000 is being discontinued within the Japanese and European markets, and sooner or later different sportbikes could also be produced for trackday use solely. What’s driving such adjustments?

Euro 5 emission requirements cuts the restrict for unburned hydrocarbons (UHC) within 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 manner into a motorbike engine’s exhaust stream in a wide range of methods:

  1. The compression stroke pushes small volumes of fuel-air combination into piston-ring&nbsp; and head-gasket crevice volumes, from which they later emerge to hitch the exhaust stream as UHC.
  2. Some gas is absorbed into cylinder-wall oil movie or combustion-chamber deposits and later evaporates from them.
  3. Many suggest that combustion is quenched by shut proximity to cooler metallic surfaces.
  4. Recent air-fuel combination short-circuiting from the consumption to the exhaust in the course of the valve-overlap interval.
  5. Poor oil management by piston rings and valve-guide seals.
  6. Incomplete combustion in slower-burning wealthy or lean combination zones.
  7. Precise misfire.
  8. Evaporation of gas from the gas tank.

Of the above, 1 and a couple of are most vital, whereas 5, 6, and seven, and eight are both already minimized by present applied sciences or are issues of engine upkeep. No. 3 stays to be confirmed, whereas 4 relies upon upon the size of the valve-overlap interval (high-power racing or sport engines have a tendency to make use of longer overlap durations).

Piston-Ring and Head-Gasket Crevice Volumes

To seek out the overall size sealed by piston rings or head gasket, we take the circumference of 1 cylinder (the gap round it—𝜋 [3.1416] occasions cylinder bore) multiplied occasions the variety of cylinders.

To get a particular measure by way of sealed size per unit of displacement, we will then divide this quantity by the engine’s displacement in liters.

The most recent GSX-R1000 has 76mm cylinder bores. Since 𝜋 is roughly 3.14, the sealed size for one cylinder is 76 x 3.14 = 238.8mm, and the overall for 4 cylinders is 4 x 238.8 = 955mm.

As a result of the Suzuki’s engine displacement is 999.8cc, our sealed size per liter for this engine is about 955mm, or about 37 inches.

Now, let’s check out an engine designed to fulfill Euro 5 from the identical firm: Suzuki’s lately launched parallel-twin 776. The bore and stroke are 84 x 70mm, with the circumference of 1 cylinder 84 x 𝜋 = 264mm. For the entire engine the determine is twice this, or 528mm. To transform this quantity into seal size per liter, we divide by 0.776 (the engine’s displacement as a fraction of a liter) and provide you with 680mm.

This quantity is simply 71 p.c as a lot because the GSX-R1000′s seal size per liter, which supplies the extra trendy engine’s builders a head begin in controlling UHC emissions. The much less complete size of piston-ring and head-gasket crevice quantity, the much less UHC can disguise from combustion there, solely to emerge later within the cycle to movement out with the exhaust.

The actually robust variable right here is the variety of cylinders. Altering from 4 cylinders to 2 reduces the sealed size loads. Altering from the GSX-R1000′s big-bore/short-stroke design to the 776′s much less excessive bore/stroke does cut back sealed size as properly, however by a lesser quantity.

Decreasing the Variety of Cylinders

To additional perceive how highly effective lowering the variety of cylinders is in lowering sealed size per liter, let’s think about changing the newest GSX-R1000 4 right into a twin having the identical bore/stroke ratio. The current engine has a bore/stroke ratio of 76/55.1 = 1.38. A 1,000cc twin with the identical ratio would have bore and stroke of 95.8 x 69mm, giving us a ring- and gasket-seal size of 95.8 x 𝜋 x (variety of cylinders) = 602mm, a considerable discount from the four-cylinder’s 955mm.

However the end result—a twin with a for much longer stroke—would by no means have been dependable on the rpm required to offer GSX-R1000 energy.

Assembly the Euro 5 UHC Restrict

To extend the GSX-R1000′s horsepower lately and allow it to succeed in greater revs with out subjecting its pistons to damaging ranges of piston acceleration, Suzuki needed to take the alternative method. The engine’s stroke was shortened and its bore enlarged greater than as soon as. Twenty years in the past the bike’s bore and stroke had been 73 x 59mm, after which 10 years in the past that was modified to 74.5 x 57.3mm. BMW, in searching for to make its four-cylinder S 1000 RR engine king of the hill, went even additional—to 80 x49.2mm. When it comes to sealed size per liter, the BMW is at 80 x 𝜋 x (variety of cylinders) = 1,005mm per liter of displacement.

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

How does a producer resolve whether or not or to not put money into the R&D essential to develop a given mannequin to fulfill Euro 5 emissions? It is a easy value/profit calculation.

If massive sportbikes had been nonetheless promoting in giant numbers, earnings from their continued sale would possibly pay the R&D invoice. However the current depressed gross sales of highly effective sportbikes make it most unlikely that R&D {dollars} spent to make GSX-R1000 meet Euro 5 would earn a revenue.

Higher to discontinue the mannequin and make investments the R&D into new designs which have higher gross sales prospects. These new designs may be made to fulfill Euro 5 extra simply—and at decrease value. Designs akin to these with fewer cylinders, smaller bores, and longer strokes.

Euro 5 and Valve Overlap

Lack of recent air-fuel combination throughout valve overlap has been one other main contributor to UHC emissions in high-performance engines. Valve overlap has been helpful in boosting efficiency, offering a “window” by way of which a low-pressure wave within the exhaust pipe, returning to the cylinder throughout this era, can suck out exhaust gasoline remaining above the piston when it’s near TDC, and may then pull into the cylinder recent cost by way of the just-opening consumption valves.

In impact, lengthy overlap can enable tuned exhaust-wave motion to begin the consumption stroke even earlier than the piston has began to descend. The draw back: The longer each consumption and exhaust valves are open, the higher the time throughout which recent cost getting into the cylinder from the consumption valves could also be misplaced out the just-closing exhaust valves. Shorter overlap means fewer emissions.

Different Methods To Obtain Excessive Efficiency

If all of this sounds just like the heavy hand of Huge Brother coming down exhausting on our two-wheeled enjoyable, let me recommend that new methods to ship helpful efficiency are leading to a minimum of as a lot enjoyable as earlier than, with an elevated sense of management and safety. Compelled to search out different methods to spice up efficiency brought on by shorter valve overlap, engineers at the moment are making up for the loss by lifting the consumption valves quicker and better. The ensuing engines have flatter, wider torque curves than the long-overlap designs. One other plus: Such engines are simpler to journey properly.

The uncooked energy of the earlier period’s sportbike engines got here at a value in weight: These literbikes had been really energy stations on wheels that frankly frightened a lot of their riders. The style by which these engines made their energy—principally up excessive within the rpm vary—made them extra demanding to journey, requiring fixed consideration to the tachometer to maintain the engine singing in its finest vary.

Decrease weight and wider energy make extra lately designed bikes simpler to journey properly, with much less deal with tach watching. The chassis and suspension additional profit from the newest in these applied sciences, returning enhanced responsiveness. Digital rider aids have now matured so their results are much less intrusive. The underside line is that it has turn out to be simpler to get the efficiency we would like from this new sort of bike as a result of we will safely use extra of what’s there.

I’m with those that beloved the excessive track of four-cylinder engines at peak revs. I additionally rejoice the flexibility of our trade to ship what we would like in efficiency whereas assembly exhaust emissions requirements made crucial by the current world inhabitants of eight billion individuals.