Renault Energy’s Sport F1 hybrid Power Unit; competing with intelligent energy management, not just power
|Exploded view of the parts of the power unit: 1.6l turbocharged V6 engine; battery; turbocharger; MGU-K; and MGU-H. Click to enlarge.|
Renault’s Energy F1-2014 hybrid Power Unit (earlier post), designed and developed for the FIA Formula One World Championship this year, is ready for the track; the 2014 season kicks off in Australia on 16 March. New FIA regulations are driving energy efficiency levels higher this year, with two types of energy propelling the cars. The internal combustion engine will produce power through consumption of traditional carbon-based fuel, while electrical energy will be harvested from both exhaust and braking by two discrete motor generator units. Teams and drivers will balance the use of the two types of energy throughout the race.
This year, the power unit is divided into six separate elements: Engine (ICE); Motor generator unit-kinetic (MGU-K); Motor generator unit-heat (MGU-H); Energy store (ES); Turbocharger (TC); and Control electronics (CE). The combination of the 1.6-liter turbocharged direct injection V6 engine (maximum speed of 15,000 rpm) with the two motor generator units and battery energy store delivers combined maximum power output of 760 bhp (567 kW), on a par with the previous V8 generation.
There is by regulation a double restriction on fuel consumption: fuel quantity for the race limited to 100 kg (-35% from 2013) with fuel flow rate limited to 100 kg/hr max (unlimited under V8 regulations). Cars will therefore need to use both fuel and electrical energy over one lap.
Power unit. The Renault Energy F1 V6 engine has a displacement of 1.6 liters and will make around 600 bhp (447 kW), or more than 3 times the power of a Clio RS. Because of the turbocharger, the pressures within the combustion chamber are almost twice as much as the earlier V8. The crankshaft and pistons will be subject to massive stresses and the pressure within the combustion chamber may rise to 200 bar. Renault notes that the pressure generated by the turbocharger may produce a ‘knocking’ within the combustion chamber that is very difficult to control or predict.
According to Renault, the option of implementing cylinder deactivation to improve efficiency and driveability through corners still remains.
Some of the exhaust gas energy recovered by the turbocharger will be passed on to the MGU-H and converted to electrical energy that will be stored and can later be redeployed to prevent the turbo slowing too much under braking.
On conventional turbo engines, a wastegate is used in association with a turbocharger to control the high rotation speeds of the system. It is a control device that allows excess exhaust gas to by-pass the turbine and match the power produced by the turbine to that needed by the compressor to supply the air required by the engine. On the Renault Energy F1, the turbo rotation speed is primarily controlled by the MGU-H; however a wastegate is needed to keep full control in any circumstance (quick transient or MGU-H deactivation).
Connected to the turbocharger, the MGU-H acts as a generator, absorbing power from the turbine shaft to convert heat energy from the exhaust gases. The electrical energy can be either directed to the MGU-K or to the battery for storage for later use. The MGU-H is also used to control the speed of the turbocharger to match the air requirement of the engine (eg. to slow it down in place of a wastegate or to accelerate it to compensate for turbo lag.) Very high rotational speeds are a challenge as the MGU-H is coupled to a turbocharger spinning at speeds of up to 100,000 rpm.
The MGU-K is connected to the crankshaft of the internal combustion engine. Under braking, the MGU-K operates as a generator, recovering some of the kinetic energy dissipated during braking. It converts this into electricity that can be deployed throughout the lap (limited to 120 kW or 160 bhp by the rules). Under acceleration, the MGU-K is powered from the Energy Store and/or from the MGU-H and acts as a motor to propel the car.
Heat and kinetic energy recovered can be consumed immediately if required, or used to charge the Energy Store, or battery. The stored energy can be used to propel the car with the MGU-K or to accelerate the turbocharger with the MGU-H. Compared to 2013 KERS, the ERS of the 2014 power unit will have twice the power (120 kW vs 60 kW) and the energy contributing to performance is ten times greater.
The battery has a minimum weight of 20kg to power a motor that produces 120kW.
The presence of an intercooler (used to cool the engine intake air after it has been compressed by the turbocharger, and thus absent in the normally aspirated V8 engines), coupled with the increase in power from the energy recovery systems makes for a complicated integration process since the total surface area of the cooling system and radiators has significantly increased over 2013.
Operation. Under acceleration (eg. down the pit straight) the internal combustion engine will be using its reserve of fuel. The turbocharger will be rotating at maximum speed (100,000 rpm). The MGU-H, acting as a generator, will recover energy from the exhaust and pass to the MGU-K (or the battery in case it needs recharging). The MGU-K, which is connected to the crankshaft of the ICE, will act as a motor and deliver additional power to pull harder or save fuel, dependent on the chosen strategy.
At the end of the straight the driver lifts off for braking for a corner. At this point the MGU-K converts to a generator and recovers energy dissipated in the braking event, which will be stored in the battery.
Under braking the rotational speed of the turbo drops due to the lack of energy in the exhaust which, on traditional engines, leads to turbo lag. This phenomenon occurs when the driver re-accelerates: Fuel injection starts again and generates hot exhaust gases which speed up the turbo, but it needs time to return to full rotational speed where the engine produces 100% of its power. To prevent this lag, the MGU-H acts as a motor for a very short time to instantaneously accelerate the turbo to its optimal speed.
|Energy flow. Click to enlarge.|
Over the course of the lap, this balance between energy harvesting, energy deployment and (carbon) fuel burn will be carefully monitored.
The use of the two types of energy needs an intelligent management. Electrical energy management will be just as important as fuel management. The energy management system ostensibly decides when and how much fuel to take out of the tank and when and how much energy to take out or put back in to the battery.
The overall objective is to minimize the time going round a lap of the circuit for a given energy budget. Obviously, if you use less energy, you will have a slower lap time. That’s fine. However, what is not fine is to be penalized more than the physics determines necessary. In the relationship between fuel used versus lap time, there is a borderline between what is physically possible and the impossible—we name it ‘minimum lap-time frontier’.
We always want to operate on that frontier and be as close to the impossible as we can. The strategy is subject its own limits, namely the capacity of the PU components and the Technical Regulations. The power output of the engine subject to its own limits, plus MGU-K power and the energy the battery can deliver to it are all restricted by the rules. It is a complex problem. The solution is therefore determined by mathematical modeling and optimization—we call it ‘power scheduling’.
As a result, there will be a complex exchange of energy going on between the components in the system network, at varying levels of power over a lap. This is completely invisible to the driver as it is all controlled electronically by the control systems. The driver will be able to feel it but no driver intervention is normally required, so they can concentrate on the race in hand. Of course, there will be certain driver-operated modes to allow him to override the control system, for example to receive full power for overtaking. Using this mode will naturally depend on the race strategy. In theory you can deploy as many times as you want, but if you use more fuel or more electrical energy then you have to recover afterwards. The ‘full boost’ can be sustained for one to two laps but it cannot be maintained.—Naoki Tokunaga, Technical Director for new generation Power Units
The fact that the driver does not control the balance between fuel and energy does not lessen the involvement of the driver in any way, and in fact the job will be more complicated than in previous seasons, Renault suggests. The driver will still be fighting the car to keep it under control during hard braking, managing braking to avoid understeer into a corner, applying delicate control over the throttle pedal mid-corner, sweeping through complex corners, throwing the car into high speed corners. In terms of driving style, there may well need to be some adjustments.
The throttle response will be different so the driver will need to readjust for this. Effectively, once the driver applies full throttle, the control systems manage the power of PU, with the aim to minimize the time within the given energy. However full throttle no longer means a demand for full engine power. It is an indication to the PU given by the driver to go as fast as possible with the given energy. He will still need to adjust for the different ‘feel’ of the car with the energy systems. In essence, engine manufacturers used to compete on reaching record levels of power, but now will compete in the intelligence of energy management.—Naoki Tokunaga