Why are there are less air-cooled motorcycles now-a-day you would ask. It all comes down to exhaust emissions. Any new production motorcycle that go's on sale in Europe has to meet a set of standards governing the gases that come out of its exhaust. The current (European) limits, known as Euro 3, were introduced in January 2006. A decade later the even-tougher Euro 4 regulations are looming – and this is what's behind the recent decline in air-cooled motorcycles. Probably a few readers will say, hold on we're in Thailand and what have European exhaust emission regulations to do with us, the thing is, the Thai government is following the European exhaust emission standards. Officially the current Thai exhaust emission standard is more tough than the Euro 3 standard, and is therefore 'unofficially' called the Euro 3+ standard.
Some will probably be asking, will the Euro 4 emission rules ban air-cooled engines. No, not specifically, but the strictness of Euro 4's new limits will make it a lot harder for them. The new rules aren't about picking on air-cooling specifically, just enforcing a blanket reduction in the three measured pollutants: hydrocarbons (HC), nitrogen oxides (NOx) and carbon monoxide (CO). The two main reasons why it is harder for an air-cooled engine to pass an emissions test is because temperature control and warm-up time. Firstly, it's harder to measure and manage the temperature around an air-cooled combustion chamber. A liquid cooling helps keep the temperature uniform across the engine, but an air-cooled engine heats up from the exhaust port. The resulting temperature gradient across the engine block is much more unpredictable – it depends on how the motorcycle is being ridden. Knowing the exact temperature around the combustion chamber is crucial for being able to calculate the correct amount of fuel for each cycle.
The second issue is warm-up time. Emission tests start with the motorcycles cold, and until engines and catalytic converters are brought up to temperature they're very inefficient. It's even possible for a engine to overstep the limits for the whole test cycle during the warm-up phase alone. Thermostats on liquid-cooled engines shut off the engine from the radiator, helping them warm up faster. Air-cooled engines don't have this luxury.
Some will probably say that modern air-cooled motorcycles already have electronic fuel injection and catalysers, like the Ducati Scrabler and BMW R Nine T. The thing is three-way catalytic converters help to remove HC, NOx and CO – or, more accurately, convert them into less-poisonous compounds such as water and carbon dioxide. However, the chemistry that goes on inside a catalytic converter relies on the incoming exhaust gas having and air-fuel ratio within a fairly narrow window. If the exhaust gas is too lean, NOx won't be reduced as efficiently. If the gas is too rich, then HC and CO emissions go up instead. The whole process is a delicate balancing act, and it relies on putting exactly the right amount of fuel into the engine to begin with.
This is why catalytic converters require closed-loop fuel injection. Sensors measure the oxygen content of exhaust gas, then feed this back to the ECU to adjust the amount of fuel being delivered.
But while this technology does reduce emissions, even liquid-cooled engines have needed all of it just to pass Euro 3 standards for the past decade. Euro 4 is going to be even tougher, and being air-cooled simply puts an engine at a significant disadvantage.
Under Euro 4 exhaust emission rules the limits for all three pollutants will be roughly half what they are today. And given the current levels are already around a third of what was allowed before 2006, it's clear just how dramatically emission rules have changed in an incredibly short space of time.
In the European Union the Euro 4 exhaust emission rules will be introduced on the 1 January 2016, and it's expected that Thailand will, after it studied the new rules, introduce a “Thai” Euro 4 exhaust emission standard within a year. Of course if you buy a air-cooled motorcycle today you will not have any problems with your registration of exhaust emission standards as the new Thai “Euro 4” exhaust emission standard will only apply for motorcycles registered after the rules are introduced.