What is engine downsizing, and why should you care?

Not so long ago, you could pretty much tell how powerful and fast a car was, just by the size of its engine. In fact, it was such a surefire method, manufacturers would – and some still do - proudly emblazon the engine size on the boot for all to behold. As a result, Bob on his weekly trip to the golf club in his “2.0” Escort, knew he clearly had a superior job to Kevin who’s shiny company car was a measly “1.6”.

But, with the perpetual march of technology, engine size is no longer the be all and end all when it comes to power. Engines have got smarter than that, and marketing folk are replacing engine badges with rousing names and logos. So, what’s the situation with engine ‘downsizing’, how does it work, why has it happened, and where will it all lead?

What is engine ‘downsizing’?

Put simply, if a car has a smaller capacity engine than its predecessor, but can still do the same job or has even better performance, then we call it ‘downsizing’. A good comparison is aeroplanes. You could keep adding bigger propeller engines to a passenger plane, but with the performance gains, they would become hugely thirstier and very inefficient. Add jet engines instead, and they are faster, smaller and make better use of their fuel.

One of the best-selling examples of ‘downsizing’  is Ford’s 1.0-litre EcoBoost petrol engine, which is available with outputs of 99bhp, 123bhp and 138bhp, meaning it effectively replaces 1.4, 1.6 and even some 1.8-litre engines from the past. And, it’s not only fitted in small, lightweight models like the top-selling Fiesta, but can be found in the B-MAX, C-MAX and Grand C-MAX, MPVs designed to carry up to seven occupants. Even the EcoSport crossover and van-based Tourneo Connect and Grand Tourneo Connect with their huge load capacities are offered with the tiny 1.0-litre EcoBoost motor.

Of course, it’s not just Ford, but almost every manufacturer who now offers ‘downsized’ engines. The Volkswagen Group has its own 1.0-litre and 1.2-litre engines available in models as large as the Touran MPV, while manufacturers like Alfa Romeo, Fiat, Kia, Hyundai, Peugeot, Citroen, Renault, Nissan, Suzuki and Toyota similarly offer small and punchy motors.

How does it work?

Engine designers are finding better ways to extract more power from smaller amounts of fuel. An internal combustion engine works by igniting fuel and using the following explosion to push the piston up and create a driving force. To ignite, fuel needs to be mixed with oxygen, so downsized engines work best if as much is present as possible. The best way of achieving this is to use a turbocharger, which forces much more oxygen into the engine cylinder, resulting in a bigger and more efficient bang.

The turbo is essentially an enclosed fan, which uses exhaust gas from the engine to spin and create its boost and pressurise the engine. This used to result in a power lag (where its benefits weren’t felt until higher revs were reached), but smaller and clever variable geometry turbochargers which get going more quickly, have dramatically reduced this effect.

Other advances include the way the fuel is introduced into the engine, with powerful computers and software controlling fuel injection to the millisecond and allowing the engine to shut down in traffic where stop and start is fitted.

Why do we have ‘downsizing’?

Over the years, government legislation has been introduced to control the emission levels of the cars we drive. Going hand-in-hand with this is taxation, which in the UK means you will pay more Vehicle Excise Duty (VED) annually if your car emits higher levels of CO2.

So, if we look at the Ford example, the Fiesta 1.25-litre petrol with 81bhp can manage 54.3mpg and emits 122g/km of CO2, while it has enough power to accelerate from 0-62mph in 13.3 seconds. The smaller-engined Fiesta 1.0-litre EcoBoost with 99bhp returns 65.7mpg, emits 99g/km of CO2 and can sprint from 0-62mph in 11.2 seconds.

The 1.25-litre is placed in tax band D and costs £110 in annual road tax, while the 99g/km emissions of the 1.0-litre means it sits in tax band A, with free road tax for cars emitting less than 100g/km of CO2.

Sound too good to be true? Well, it’s all just good old physics and clever engineering, but there is a downside. The extra equipment and development required to make an advanced turbocharged engine makes the EcoBoost Fiesta more expensive to buy, and it could also cost more to repair as the years go by.

Where will it all lead?

The current European legislation manufacturers need to adhere to is called Euro 6, and it came into force in September 2015. Its main impact was to reduce the level of accepted NOx emissions from diesel vehicles from 180mg/km to just 80mg/km, while the amount for petrol vehicles stayed at 60mg/km. 

With tougher restrictions coming into force, engines will need to become more and more efficient. We’ll have to wait and see what manufacturers come up with, but there are a few concepts which hint at the future.

Audi is looking to introduce electric turbocharging, which has a few advantages over the traditional type. Firstly, it can provide maximum boost almost instantly, with no need for the engine to be up to speed, and secondly, the exhaust can be made more efficient without a turbo scavenging energy.

BMW is also investigating water injection, which mixes a tiny amount of water with the fuel and air in the cylinder. Now, this might sound odd, but water has a cooling effect, making air denser and the combustion process more efficient, resulting in power and efficiency gains.

But, probably the biggest play for ‘downsized’ engines will be in hybrid vehicles. Their high performance in a small package – the base of the 1.0-litre EcoBoost engine can fit on a piece of A4 paper – is ideally suited to being squeezed under the bonnet of a small hybrid car, next to an electric motor and with a battery pack under the floor or boot. Using a combination of combustion and electric power is likely to be the only way to meet ever tightening emissions legislation in the near future.