The availability – albeit in small numbers – of cars using hybrid petrol/electric drivetrains creates the possibility of doing unique modifications – ones that in the history of cars, have literally never before been done. In this world-first story we modify the regenerative braking capability on a Toyota Prius to give greater braking capability in light braking applications. It’s a mod that needed no brake fluid, no rags, created no brake dust and yet makes a major difference to how the car brakes!
So what the hell is ‘regenerative braking’?
Regenerative braking ("regen") occurs only in vehicles that use electric power. During braking, the vehicle’s electric motor is operated as a generator, pushing power back into the battery. (Regen braking is used on electric trains, where it feeds power back into the overhead wires. And the electric aircraft pusher shown at left also uses regen braking!)
By using regen rather than friction brakes, the power that’s usually wasted in heating the brakes gets turned into usable energy that is stored for later use.
In a pure electric car, the use of regen braking extends the range, while in hybrid petrol/electric cars, it improves fuel consumption and emissions because the petrol engine doesn’t need to be used as much.
However, regen braking isn’t strong enough to do all the braking, so cars with regen braking use conventional hydraulic friction brakes as well. And this is where it starts getting tricky. If there are two completely separate braking systems at work, how are they combined in their outcome so the driver has to press only the one brake pedal?
In the pure electric Solectria that we drove back in 2001 (The Solectria DW Electric Car), the regen braking was operated by the throttle. Push the throttle past half-way and the car accelerated; lift it above that halfway point and the car decelerated with regenerative braking. The hydraulic brakes were operated with a conventional second pedal, being needed in most cases only for the last few metres before the car came to a complete stop. We thought the Solectria’s combined regen braking/accelerator pedal worked very well, but it was certainly a different control approach to conventional cars.
To keep the cars feeling as much like other cars as possible, the hybrids from Toyota and Honda have both the regen and conventional brakes controlled by the one brake pedal. In the first part of its travel, the brake pedal operates the regen brakes alone, then as further pressure is placed on the pedal, the friction brakes come into play as well. The current Civic Hybrid mixes the two brake modes together imperceptibly, whereas the first model Prius, for example, has more of a two-stage pedal.
Incidentally, it’s not only when the brake pedal is being pushed that the hybrids regen. When the throttle has been fully lifted, a gentle regen automatically occurs. In the Hybrid Civic it’s like you’re decelerating in (say) third-fourth gear in a manual gearbox car, while in the Prius models it’s less strong.
The Point of the Modification
The regen in my ’99 Prius has always seemed to me to be less than fantastic. Firstly – and perhaps because I am running Kevlar pads and slotted discs – the friction brakes seem to do nearly all the work. When braking at slow speeds, the friction brakes can be heard to be working (the hi-po pads have a slightly graunchy sound), and at higher speeds, the pedal pressures are very much of the sort that occur with standard hydraulic-only braking systems.
In short, it has never felt much like the regen is doing a lot.
The Prius runs a centre dash LCD that shows how many watt-hours of regen have occurred each 5 minutes. These are indicated by small ‘suns’ that appear on the display – each sun being indicative of 50 watt-hours. (So when a sun appears, enough power has been put back into the battery to run a 50-watt light bub for an hour – it’s not a trivial amount.) But in my driving, seeing a lot of suns is rare - indicative that the regen isn’t contributing much. (The battery level also gives an indication of how much regen is occurring - the battery level can be seen on the right in this display.)
So it seemed to me that if the regen could be tweaked to do a greater proportion of the braking work (especially in light braking), fuel economy would benefit, the braking would be smoother, and it would take lighter pedal pressures.
The Prius is a car with stunning engineering.
So it comes as no surprise to find that the system that integrates regen and hydraulic braking is complex. (Click on the diagram to enlarge it.) In fact, it’s the ABS ECU that handles regen braking as well as ABS functions, sending a signal to the hybrid ECU to tell the hybrid ECU how much regen to impose. But how does the ABS ECU know what to do?
Rather than measuring brake pedal travel (which could vary with pad wear, etc), the system uses pressure measuring sensors to detect master cylinder pressure. The higher the master cylinder pressure, the harder the driver is pushing on the brake pedal.
If the driver is pushing only gently, the piston displacement will be small and so the hydraulic brakes will be only gently applied. In this situation, the ECU knows that the driver wants only gentle deceleration and so instructs the hybrid ECU to apply only a small amount of regen. However, as master cylinder pressure increases, so does the amount of regen that can automatically be applied.
(In fact, there are four pressure sensors in the braking system and two pressure switches – but it’s the master cylinder pressure sensor that is most important.)
Modifying the System
So if the amount of regen that occurs is largely dictated by the output of the master cylinder pressure sensor, what about intercepting and altering this signal? That way, the ABS ECU would think that there was more master cylinder pressure than was actually occurring, so resulting in more regen being applied. Since the actual hydraulic pressure going to the brakes would be unchanged, there’d be a greater proportion of regen braking in the mix.
The voltage output of the pressure sensor ranges from about 0.4 – 3 volts, rising with increasing pressure. So if a small voltage could be added to this signal, the ECU should respond with more regen braking. But if this was done, would it detect a fault condition? The workshop manual states that a fault will be detected if the voltage from the sensor is outside of the range of 0.14 – 4.4V, or if the voltage output of the sensor is outside a certain ratio to its nominally 5V supply voltage. Further, the latter is checked when the brake switch is off (ie brake pedal is lifted).
In other words, the voltage needs to be within a certain range and in some cases this is checked with the brake pedal not being used.
Leaving out a lot of the ECU connections, here’s what the master cylinder pressure measuring system looks like. From top, there’s the voltage signal from the sensor, the regulated 5V supply to it, the input from the brake light switch (12V when the brakes are on), and the earth connection. (Note that for the following circuit, it doesn’t matter which regulated 5V supply and earth connections on the ECU are used – there are several.)
As indicated above, what we want to do is to lift the voltage output of the master cylinder pressure sensor, especially at low sensor output levels. This is easily achieved with a single 100 kilo-ohm pot. If the pot is wired between the output of the sensor and a regulated 5V, and the wiper of the pot is then connected to the ECU, the voltage that the ECU sees can be varied from a constant 5V (not wanted!) right through to the standard signal. If the wiper is adjusted so that it’s just a little way towards the 5V end, a small voltage will be added to the signal. Note that to allow for the required fine adjustments, a multi-turn pot should be used.
But what about the way the ECU checks the sensor output voltage when the brakes are off? In that case, it might spot that the output voltage of the sensor always appears to be a bit high. The easy way around this is to add a relay that bypasses the pot whenever the brake pedal is released. This can be achieved with a low current SPDT 12V relay. As shown here, whenever 12V is available on the brake light circuit, the relay opens, sending the signal through the pot. But when the brake lights are off, the relay closes and so the sensor input voltage to the ECU is effectively standard. As a result, the system works as standard until the brake light switch comes on, whereupon whatever adjustment has been set on the pot immediately comes into effect.
This graph of the input and output voltages at different braking efforts shows what happens. When the brake pedal has not been pushed, the input and output voltages are the same. When the brake pedal has just been tripped, the output voltage rises. The amount that the output is greater than the input progressively reduces as braking effort increases, until at heavy braking load the signal is back to standard.
Wiring it Up
With a workshop manual it’s straightforward to find the right wires – but always double-check with a multimeter and make sure the colour-codes of the wires match those shown in the manual. The pot and relay (the latter salvaged from an old ABS controller!) were mounted on pre-punched board.
The first step was to make sure that the relay clicked in and out with movement of the brake pedal. Next, the pot was set so that its wiper was right at the ‘sensor’ end and a multimeter was used to check that the signal going into the ECU was the same whether the relay was tripped or not. The car was test-driven and the braking system worked exactly as standard... at this stage, just what was wanted!
Next, the pot was adjusted a little so that the output signal of the sensor was dragged upwards. Driving of the car then showed a fascinating outcome. When the brake pedal was pressed just far enough that the brake light switch tripped, the car would immediately decelerate with regen braking alone. It was quite a clear and distinct feeling – no friction brakes operating, but the car rapidly slowing. In fact, at this level of pot adjustment, too rapidly slowing...
The pot was then adjusted back a little and further driving undertaken.
As finally set, the very light pedal brake pressure voltage at the ECU input is lifted from 1V to about 1.15V – just a 15 per cent increase at this end of the sensor’s output. This results in a clear deceleration when the pedal is lightly pressed, and much stronger regen than normal as the pedal is pressed harder. At high braking efforts, the behaviour of the car is near standard – it’s in very light braking where there’s a clear difference.
But I’ll be frank – I am not sure that everyone would like the end result. Why? Well, that takes some explanation.
With a purely hydraulic braking system, if they so desire, the driver can brake extremely gently. In fact, perhaps only imperceptibly slowing the car. However, with the modified regen system, the minimum braking decel that can be achieved is now noticeably more than the minimum with a purely hydraulic system. That is, if you want to slow down, you put your foot on the brake pedal. If you don’t want to slow down, don’t brake – cos if you do, for the same light pedal pressure you’ll slow at a quicker rate than in a conventional car.
So extremely gentle braking, of the sort perhaps where in a conventional system you’d just barely be touching your foot on the brake pedal, is no longer possible. Once that switch is tripped, braking is occurring. This is because when the relay is activated by the action of the brake pedal, the ECU immediately thinks that the brakes are being applied more strongly than they really are – and so gives you the extra regen braking.
But it’s important that this minimum braking threshold not be overstated. Testing with an accelerometer showed that when the throttle is lifted (without any braking occurring), the deceleration from 100 km/h is about 0.1g. When the brake pedal is pressed at its modified minimum value (that is, the relay is just tripped), the deceleration increases to 0.11g. At 60 km/h, a throttle lift results in 0.08g deceleration, while minimum braking results in 0.12g decel. (As a guide, about 0.8g indicates very hard braking.)
This graph compares the figures - a throttle lift typically giving about 0.1g decel, minimum regen braking about 0.11g, moderate regen braking about 0.2g, and a hard emergency stop resulting in braking of (at least) 0.8g.
Logging the input signal from the sensor and the modified output signal to the ECU shows that over a 6 minute hilly urban drive, the average value of the sensor signal was 0.507V and the average value of the modified output was 0.562V. However, as the lower trace shows, the modified output of the sensor has a lot more ‘area under the curve’, a better indication of the changed feel on the road.
And the benefits?
Firstly, the regen braking is clearly doing much more of the braking work. Or, to put it another way, much less energy is being wasted in the brakes. This can be both seen in the display of watt-hours regenerated (there are more suns than usually achieved on this stretch of road), and also in the feel of the car. The regen braking is smooth and effortless, slowing the car substantially before the brake pedal is moved a little further to activate the hydraulic brakes and bring the car to a halt.
It’s hard to assess overall urban fuel consumption, but in some 5 minute increments it has improved by 30 per cent.
The wear of the hydraulic brakes and pads will also be substantially lessened. (These brakes still get used in anger down long hills, though – as the battery fills, regen progressively reduces its braking action and the hydraulic brakes then do more and more of the work.) The regen also switches off at very low speeds, so the discs still stay shiny.
Sure, we know that for nearly everyone reading this story, the described modification will be irrelevant – they won’t be able to do it on their own car. But firstly, this mod shows that whatever the direction that future cars takes, tweaks will always be able to be performed.
And secondly, if you’re driving a conventional car, start thinking: why am I paying more at the bowser because my car wastes all braking power in heat? The latter is a damned good question!