Last week Birmingham-based start-up Aeristech secured £500,000 to develop its electromechanical turbocharger.
This week, CEO Bryn Richards revealed the firm's technology - which exploits state-of-the-art motors and generators - to Electronics Weekly.
Turbochargers are well known for adding power to cars.
Less well known is that they can save fuel although, inconveniently, the large turbos needed for maximum fuel economy have so much turbo lag (see below) that cars become unpleasant, or even difficult, to drive.
"The whole industry says turbos can achieve 25% petrol savings," Richards told Electronics Weekly. "But the result is just not good enough to put in a vehicle."
Aeristech's answer is to replace the mechanical coupling shaft (see below) within a turbocharger with a generator, motor, and energy storage device.
With this arrangement, dubbed Hybrid Turbocharger Technology [HTT], acceleration is instant because the motor can drive the compressor immediately acceleration is required - using power from the energy storage device.
The exhaust-driven generator refills the storage device once the engine has speeded up.
In effect, the driver gets all of the fuel savings, and none of the drivability problems.
Decoupling the turbine and compressor has another advantage, according to Richards.
"Often the turbine is compromised by designing it for low inertia to reduce lag," he said. "We can design our compressor purely for efficient steady-state operation."
Overall, the motor-generator has higher losses than a mechanical shaft, but claims Aeristech these are offset by gains possible once the turbine and compressor are decoupled. "Efficiency of our system is about the same as a normal turbo; in the region of 70% from exhaust to intake," said Richards.
What makes electromagnetic decoupling practical, rather than overly bulky and heavy, is the high speed at which suitable turbines and compressors have to rotate - up to 100,000rpm.
"As a rule of thumb, 20% of the power of an engine can be passing across the turbocharger, so for a 100kW [130bhp 1.4-1.5 litre] engine it is about 20kW," explained Richards. "Motor and generator size is proportional to torque, so a very high speed 20kW machine can be about the same size as a fist."
Both machines are water-cooled, dumping heat into the engines cooling system. "And we have some clever design tricks to protect the generator from the hot turbine," added Richards.
The electrical machines use permanent magnets, and are controlled by a proprietary algorithms.
This is not the first super-fast permanent magnet motor, Dyson introduced one for vaccum cleaners last year, but they are rare because there is an art to designing rotors that stay in one piece at 100,000rpm.
The generator replenishes the energy store after 0.5 to 1s, so it needs to hold around 20kJ for 20kW.
"Storage can be a supercapacitor, or in a hybrid vehicle it might be the main battery pack," said Richards. "It can't be the vehicle's Pb-acid battery."
The generator provides all the power that the motor needs, and could feed spare power into other vehicle systems - although it is unlikely to replace the car's alternator. "Manufacturers would worry about the one driver in a thousand that goes everywhere in fourth gear," quipped Richards.
Weight gains will be small as the motor, generator and storage device are partially balanced by a smaller engine, simpler plumbing (the turbine and compressor can be located apart), removing the inlet pressure relieving 'waste gate', and a plastic rotor in the compressor which is no longer attached to the red hot turbine.
The firm is working with car makers, aiming to licensing Aeristech technology to their supply chains.
Turbo-lag is not the only problem HTT is claimed to solve.
By introducing a variable turbine to compressor power ratio, an engine can be adjusted to best use different fuels automatically - for example: biofuels with different proportions of ethanol.
"It is a lot like when the industry came up with fuel injection in the late 80s and early 90s," said Richards. "Manufacturers got direct electronic control of fuel then, we are offering electronic control of air now."
Turbos, power, and lag
Since the earliest days of internal combustion engines, it has been known that a modest increase in inlet pressure, frequently from a crankshaft-driven pump, can give a significant increase in output power - so called supercharging.
If an inlet compressor is instead driven by a turbine in the exhaust system, the scheme is known as turbocharging.
Crankshaft-driven superchargers deliver lots of power, but little in the way of improved efficiency.
Turbochargers have more scope to improve fuel efficiency because they are essentially driven by waste energy.
A suitable turbo adds 25-30% to power output, allowing a conventionally aspirated 1.6 litre engine to be replaced by a 1.2 or 1.3 which, Aeristech CEO Bryn Richards claims, will have 25% better fuel consumption on average in a mixed driving environment.
In a constant speed application, like a ship engine, a turbocharger has few disadvantages beyond added complexity.
In a variable speed applications including cars, turbochargers have a problem.
At low speed, there is very little gas flowing through an engine.
If hard acceleration is required then, a lot of input pressure is needed just when the turbine has very little to propel it.
Eventually, after perhaps a second, the problem sorts itself out and the engine speeds up enough to produce sufficient exhaust flow for the turbo to deliver the required inlet pressure.
The delay between power demand and the delivery of desired acceleration is called turbo lag.