The briefing: are air-cooled bike like this doomed?

Published: 16 September 2015

Air-cooled engines are characterful, simple, affordable – and doomed, right? 

ut hang on a minute, haven't they been since the dawn of the water-jacket, let alone the EU? Time to get your facts straight, fast.

Q Why are there so few air-cooled bikes?

A It all comes down to emissions. Any new production motorcycle that goes on sale in Europe has to meet a set of standards governing the gases that come out of its exhaust. The current 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 bikes.

Q Is Euro 4 going to ban air-cooled engines?

A 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).

Q Why is it any harder for an air-cooled engine to pass an emissions test than a liquid-cooled engine?

A Two main reasons: temperature control and warm-up time. Firstly, it’s harder to measure and manage the temperature around an air-cooled combustion chamber. A water jacket helps keep the temperature uniform across the engine, but an air-cooled motor heats up from the exhaust port. The resulting temperature gradient across the engine block is unpredictable – it depends on how the bike 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 bikes cold, and until engines and catalytic converters are brought up to temperature they’re very inefficient. It’s even possible for a motor to overstep the limits for the whole test cycle during the warm-up phase alone. Thermostats on liquid-cooled motors shut off the engine from the radiator, helping them warm up faster. Air-cooled motors don’t have this luxury.

Q But air-cooled bikes already have fuel injection and catalysers. Isn’t that enough?

A 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 cat relies on the incoming exhaust gas having an air:fuel ratio within a fairly narrow window. If the exhaust gas is too lean (too much oxygen), NOx won’t be reduced as efficiently. If the gas is too rich (too much unburned petrol), 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 motors 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.

Q How much tougher will Euro 4 be compared with what we have now?

A Under Euro 4 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.

Q What are manufacturers doing about it?

A Largely abandoning the air-cooled Titanic for water-cooled lifeboats, it seems. The only air-cooled motor left in Ducati’s European range is the Scrambler. BMW have only the R nineT. Harley-Davidson introduced liquid-cooled cylinder heads to some of their tourers as part of Project Rushmore last year, while spy shots show Triumph are developing water-cooled Bonnevilles. Of the recognisable brands, only Moto Guzzi and Royal Enfield still have an entirely air-cooled range.

Cooler heads will prevail: BMW and Harley have introduced “precision” water-cooling systems to regulate the area around the combustion chambers. This is the engine in the R1200GS.

Q Why is this all happening now?

A Because Euro 4 is just months away. From January 1 2016, every new bike introduced to the European market will have to meet the new standards. Anything already on sale now gets a further 12 months’ grace. But from 2017 all new bikes will have to be Euro 4 compliant.

Q What about bikes already on the road?

A They’re safe – Euro 4 won’t affect them. The tougher limits will only apply to new models. Euro 4 is nothing to do with the MoT test either (which, for motorcycles, doesn’t even test emissions anyway), so there’s no need to worry about any bike you already own. Any unsold non-Euro 4 bikes still lurking in dealers towards the end of 2016 will probably be preregistered to get around the rules.

So are air-cooled bikes doomed?

No, but their future looks a lot less bright from 2017. Perhaps the most important thing will be for riders to adjust their expectations. The smaller an air-cooled engine is, or the lower the performance level demanded from it, the easier it will be to get it to pass Euro 4. The air-cooled engines that survive the next few years will almost certainly reflect this. It could be one reason why the motor in Ducati’s Scrambler is 12bhp down from when it powered the Monster 796.

A manufacturer might still be able to make a large, powerful air-cooled engine pass Euro 4, but something else would suffer instead. Its part-throttle response may be atrocious. Or it might cost a fortune in development, resulting in a price that would put off customers.

This is not a simple black-or-white issue. The long-term survival or extinction of air-cooled bikes can’t be guaranteed either way, but things will get a lot harder for them. Their longevity depends on the attitudes of both the manufacturers making them and, more crucially, the riders buying them.

Will it be more important to riders that “traditional” bikes remain air-cooled, or that they’re nicer to use? Indian’s liquid-cooled Scout and Triumph’s future Bonnevilles appear to give one answer, yet the popularity of (and investment behind) both BMW’s R nineT and Ducati’s Scrambler gives another. And it’s a question every firm will have to answer at some point in the next two years. 

Words: Martin Fitz-Gibbons