Why Inline

Updates to emission regulations in Europe might bring an end to the high performance Inline-4 era

Inline-4 engines for motorcycles may be going the way of the Dodo. It is well known that regulations largely shape the automotive and motorcycle landscape, due to ones that are coming up recently this popular engine format might be going extinct. Emissions standards are tightening around the world and leading the charge is Europe which can in effect dictate what bikes are available for the entire world.

It's expensive for motorcycle (and car) manufacturers to make different versions of the same model for all the different regions where they sell their vehicles, and much easier and financially favorable to just basically make a single version of a model that passes all regulations in the strictest countries, so it can be sold anywhere. Couple this with the likely fact that over time things like emissions regulations have been on a trend of becoming even stricter over time as environmental concerns and awareness grow. Here's a look into some of the reasons why the popular Inline-4 engines might be a thing of the past pretty soon.

Related: 10 Reasons Why The 2022 Suzuki GSX-R1000 Is Still The King of Sports Bikes

The fuss around emissions in regard to motorcycles and why the end of Inline-4 engines may be nigh all has to do with UHC or unburned hydrocarbons. There have always been limits to UHC all around the world but the strictest seems to be Europe and the new Euro 5 emissions standards squeeze things even a bit tighter leaving the Inline-4 in a very precarious position.

Hydrocarbons that don't get burned find their way to a bike's exhaust system and escape out into the open air and environment. Simple issues like the evaporation of fuel in the tank, misfires, incomplete combustion from running too lean or rich, and poor oil containment around pistons and seals are common however usually already highly mitigated by routine maintenance and current technology. There is the controversial issue of combustion being stifled by cooling from colder nearby metal surface areas, but this is highly debatable. Another source of UHC is hiccups in the air-to-fuel ratios coming from the intake to the exhaust due to the valve overlap time. The evaporation issue also comes up again, not in the fuel tank this time but in the cylinder walls and combustion chamber as some fuel gets trapped in film and deposits and eventually evaporate. Finally, the compression strokes can push fuel-air mixtures into places where it eventually makes their way to the exhaust in the form of unburned hydrocarbons. The Euro 5 emissions have cut things from .17 to .10 UHC in the exhaust and oxides of nitrogen or NOx from 0.09 gm/km to 0.06. The carbon monoxide emissions have also been cut from previously 1.14 km/gm down to 1.0 gm/km. All this may mean doom for the beloved Inline-4 engine.

Related: The 10 Best Inline-Four Cylinder Motorcycle Engines Ever Built

The problems mentioned earlier and in particular the bigger ones already have possible solutions coming up to help reduce those unburned hydrocarbons. One of the bigger aforementioned issues with UHC is piston ring and head gasket crevice sizes where nasty unburned hydrocarbons can hide.

One can measure the volume of those surface areas to get a potential idea of crevices for the gases to hide. Using the circumference of the cylinder multiplied by the engine's displacement using liters as the unit of measure will give the total length sealed by the head gasket and rings. One of the bikes believed to be a casualty of the new Euro 5 emissions is GSX-R1000 which was recently discontinued in Europe and Japan, and has a sealed length of roughly 37 inches once all the math has been done. By contrast, a bike also from Suzuki built to current Euro 5 requirements is the 776cc, which uses a parallel twin instead of the titular inline-4 engine. The 776 engine has a sealed length of about 27 inches which is significant, with around a 30 percent drop in surface area for UHC to hide. The key to reduction in the previous example is reducing the number of cylinders from four (inline-4) to two via the parallel-twin engine. This solution to meeting the demanding emissions requirements is one solid reason the inline-4 may be chugging out its last few revs.

There are pros and cons to the paradigm of the inline-4 and how things are likely to change. A big issue in meeting the Euro 5 emissions standards alongside the number of cylinders is valve overlap time. Valve overlap helps to increase functioning at high rpm by utilizing the Scavenging Effect, where the exiting exhaust pulls in fresh air and fuel. To do this, valve overlap has to have the exhaust and intake valves open simultaneously, creating more opportunity for UHD to escape and increasing emissions. Valve overlap has been optimized in inline-4 format, but overlap time has to be reduced to help lower emissions.

The drawback to long vale overlap is having to keep the rpm high, which is great for high speeds and racing but not so convenient for daily rides. Reducing the valve overlap time, decreasing engine displacement, and hence weight, all equal a more comfortable and relaxed ride, something that can be neglected if one has tunnel vision about high performance. The changes will result in lower performance by some metrics, but a less intimidating ride for riders with less experience, more comfort for all, and naturally, less pollution.

Jonathan has been enthusiastic about cars and motrocycles since he was a teenager. He learned to ride when he was 19 and loved it ever since. Passionate especially about Vintage bikes like Vincents, Indians, Harley's, Triumphs, and American cars of the muscle car era. Jonathan is currently saving for his dream car, a Silver 63' Stingray.