A hybrid turbocharger is an electric turbocharger consisting of an ultra high speed turbine-generator and an ultra high speed electric air compressor. The turbine and compressor are high-speed aeromachines, as in a conventional turbocharger. The electrical motors run at speeds in excess of 120,000 rpm and when used as generators, generate electricity at up to 98.5% electrical efficiency. High electrical efficiency is paramount, because there is no mechanical link between the turbine and compressor. In other words, hybrid turbocharger refers to aseries hybrid setup, in which compressor speed and power are independent from turbine speed and power. This design flexibility leads to further improvements in turbine and compressor efficiency, beyond a conventional turbocharger.
The designers claim that hybrid turbocharger technology (HTT) virtually eliminates turbo lag and enables engine downsizing without compromising engine performance. This means that a HTT equipped engine can save up to 30% on CO2 emissions and fuel economy compared to an equivalent naturally aspirated engine.
Turbos spend a lot of time dumping excess exhaust overboard to stop from overboosting the engine. Rather than use the wastegate (valve that bypasses the turbo) to do this these turbos turn the bearing section into a generator that harvests this energy and stores it for later use (by holding the shaft RPM constant and absorbing the excess energy trying to spin the turbine faster.) This may or may not be combined with a traditional hybrid vehicle drive system.
What gets exciting is when you reverse the flow. Put your foot down at 1500RPM with one of these and rather than waiting for the whole system to work itself up to a high enough exhaust energy to spool the turbo and develop power – the generator reverses roles and spools the compressor. Your turbocharger briefly turns into an electronic supercharger. This rapidly increases exhaust energy/mass flow and after a second or two exhaust power takes over and the motor becomes a generator again.
Net result? Turbocharger power and efficiency with 0 lag and fuel economy savings. Combine this system with a hybrid drive and prolonged high power operation can harvest serious waste energy from the exhaust that would otherwise be dumped overboard by waste gate and store it in the hybrid system for use as actual wheel torque. This is exciting stuff as this is exactly where most hybrid systems fall flat on their face – during cruise. The ability to actively capture this exhaust heat energy even during steady state operation is a huge boon both on the performance and the efficiency fronts.
There are many variants of the hybrid theme ranging from a turbo with only one component changed or modified through to one with all the components being changed or modified in some form or another, therefore there are no hard and fast rules on hybrid designs or the performance you can achieve… it is a bit like a modified engine, it looks the same from the outside but is crammed full of more expensive, more exotic material, higher flowing components on the inside.
The basic idea is to get more flow from the turbocharger at a given rotor speed… this can be achieved in various ways from larger or more efficient compressor wheels, higher flow compressor covers with a larger A/R, cut back turbine blades and larger A/R turbine housings etc… Simply “winding the boost up” is not the answer. Yes, in most cases it will give you more power but what are you doing? If you take a standard turbocharger and increase it’s boost pressure output you are doing two things that are or can be detrimental to the turbo and/or engine. Firstly, you are making the turbo work harder/spin faster to make that extra pressure. The turbine is rated to rotate at a maximum reliable speed. Spinning it faster could take you outside that zone and then you can run the risk of turbine blade failure. Running the compressor wheel faster will certainly give you more flow and pressure but at the expense of a lower efficiency and therefore a higher charge temperature. This can cause a higher likelihood of pre-detonation and limit power output. A hybrid turbo, if designed properly, will allow the extra flow and pressure to be achieved at a safe and reliable turbine speed and at higher compressor efficiency.
So, more flow means more air, hopefully at a cooler temperature, which you can then add more fuel to and more power. Many people including some tuners become obsessed with ‘boost’ whereas the reality of professional tuning is all about flow through the engine.
But be warned not all hybrids are that good, there is usually a trade off for all this performance gain… lag!!… (Excluding the latest ball bearing & Variable Nozzle Diesel turbos). It is very difficult to get improved performance/response all through the rev range with a standard journal bearing turbo and still get big output at the higher end of the rev range… that isn’t to say it can’t be done, it can be with careful design but on the whole there has to be a compromise… We will always give you honest advice, not sell you up and then have you disappointed or upset. If you’re a person who likes to shop around and you find wildly differing prices, be warned, whilst there are occasionally bargains around when buying turbochargers, with Hybrids you do pay for what you get. Find out exactly what you are being offered for your money.
The electric motors utilize permanent magnets which have a higher efficiency than standard high speed-induction motors. Induction motors induce an electro-magnetic field into a solid rotor core. The induction motor is much simpler to control, but there are substantial losses involved in generating the magnetic field in the rotor.
The HTT motor will accelerate from 40,000 to 120,000 rpm in less than 450 ms. The HTT motor control therefore requires a fast acting CPU to match the magnetic field of the stator to the changing position of the rotor.
At high engine speeds there is more energy generated by the turbine than is required by the compressor. Under these conditions, the excess energy can be used to recharge energy storage for the next acceleration phase or used to power some of the auxiliary loads such as an electric air conditioning system.
When combined with a variable geometry turbine, the back pressure on the engine can be varied according to the electrical demands of the vehicle and charge state of the energy storage medium.
Development is underway for replacing battery energy storage with a super capacitor which can be charged and discharged very quickly.
For the majority of the time the hybrid turbocharger is operating, the compressor and turbine power (not necessarily speed) will be matched. This gives an extra degree of freedom to the designer of a turbo charger impeller.
- Improved packaging by enabling the turbine and compressor to be placed in separate parts of the engine bay.
- Higher density charge air by reducing the length of intake ducts and increasing the size of the compressor wheel.
- ECU controlled boost levels will enable tighter predictive control of in-cylinder combustion.
- Similar engine downsizing benefits to a hybrid vehicle, but with far less (approx 1/7th) energy storage capacity to achieve the same level of downsizing.