“Diesel has emerged as the dominant fuel type for company cars, as a result of great fuel efficiency, performance and low cost of ownership under the government’s CO2 emissions based tax regime,” says Gerry Keaney, chief executive of the British Vehicle Rentals and Leasing Association, whose members own or fleet manage more than three million cars in the UK.
“But the diesel proportion of new registrations has been falling gradually for some time, as modern petrol powered cars have become better at delivering similar benefits, and we expect this trend to gather pace.”
In the UK, even company car buyers now see downsized petrol engines, many emitting around 100g/km CO2, as a viable, efficient alternative to diesel.
This is not just down to “anti-diesel sentiment”, says Al Bedwell, director, global powertrains at LMC Automotive. “It has more to do with petrol getting better and staging a fight-back, especially in small cars in Western Europe.”
Manufacturers such as Ford, Opel/Vauxhall, Hyundai and Volkswagen are all offering similarly downsized petrol engines these days, many emitting around 100g/km of CO2.
In Europe, diesel’s share of the market is set to drop from 53.3% of the market in 2014 to 51.5% in 2015, says Mr Bedwell, then continue sliding to 35% by 2020.
Turbo chargers are traditionally associated with diesel engines, which needed a boost to give them more oomph. They weren’t “much fun to drive” without them, says Guillaume Devauchelle, head of innovation and science at automotive technology company, Valeo.
And the relative cost of adding turbo to an expensive diesel engine was lower, he explains.
But turbos are now increasingly infiltrating petrol engines because they deliver dramatic emissions reductions and improvements in fuel economy, without sacrificing performance, says Craig Balis, chief technology officer of Honeywell Transportation Systems, the world’s largest turbo maker.
A two-litre turbo-charged four cylinder petrol engine can match the output of a three-litre naturally aspirated V6 petrol engine, he says, so “the technology we have is really a no-compromise solution”.
Turbos work by using the engine’s exhaust gas to drive a turbine, which in turn drives a compressor, which compresses air. This air is then forced into the combustion chamber where it mixes with fuel to create additional power.
This means the engine won’t have to burn so much fuel to deliver the same output.
“Our turbos for passenger vehicles have turbines that spin at 200,000-300,000 revolutions per minute (rpm), generating temperatures of up to 1,000 degrees Celsius, so the metal is literally glowing red,” Mr Balis says.
By comparison petrol engines operate at just 6,000-7,000 rpm and diesel at 5,000-6,000rpm.
To cope with such extreme speed, pressure and heat, turbos need to be incredibly robust, so Honeywell is using ball bearings and other technologies that have been developed for military aircraft by the company’s aerospace division.
The turbos are also coupled with intercoolers that cool the airflow and increases its density as it is supplied to the engine, and with oil cooling systems that prevent overheating.
Turbos are often combined with direct or indirect fuel injectors and variable valve lift or timing systems to make the process even more efficient.
Electrified superchargers, which compress air for just a few hundred milliseconds to add brief low-end torque until the turbo charger kicks in, will also hit the market in the next few months.
E-chargers, or e-turbos, will transform the driving experience, believes Mr Devauchelle, as they eliminate what’s called turbo lag – that slight delay in power boost you experience after pressing the accelerator.
“The turbo increases the engine’s maximum power. The e-charger gets you there even quicker,” he explains.
As such, e-turbos may rival established twin-turbo technology, where a small turbo takes care of the early stages of acceleration before the second turbo takes over.
The e-turbos’ batteries can be recharged in different ways, for instance by capturing energy during braking, explains Mr Hahn.
With enough electric power, e-chargers could take over more and more of the work done by the turbo.
Eventually carmakers will redesign vehicle architecture, moving from standard 12-volt batteries to higher voltage systems.
Forty-eight volt architecture is emerging in luxury cars with many electric components, but e-chargers can also run on 12-volt batteries if they are only required to deliver brief boosts, explains Mr Devauchelle.
‘Golden age of turbo’
“Petrol power is moving from naturally aspirated engines to turbo charged engines at a faster rate than ever before,” says Terrence Hahn, president and chief executive of Honeywell Transportation Systems.
“Over the next five years, we’ll go from about a third to around half the cars sold having turbo chargers, and the growth will continue,” he predicts.
“We call this ‘the golden age of turbo’.”
But there is no silver bullet as carmakers continue to grapple with ever-stricter emissions regulation, coupled with huge penalties for non-compliance.
Any number of combinations of e-chargers, turbo chargers, multi-stage boosting, fuel injection, variable valve systems, and combustion-electric hybrid technologies are being explored.
“During 30 years in the industry, I have never before seen so much diversity,” says Mr Devauchelle.
“Nobody can afford the penalties.”