Once upon a time, I took my Wehrmacht staff car to the Palo Alto service shop. As I mentioned a barely perceptible change in the feel of velvety autobox when it shifted gears, Ernesto, the all-knowing, all-seeing tech nodded: ‘Yes, we need to load a new revision of the software in your automatic transmission…’

When I mentioned this to my Monday Note boss, he jumped: ‘Why don’t you write a piece on cars becoming soft, that is an exposé of the total invasion of software in today’s cars?’
We’re both geeks, you see. And, yes, I could sing the praise of Toyota’s PSD (Power Split Device), the clever Continuously Variable Transmission (CVT) used by the Prius. (That type of transmission was invented in the 60s by TRW, Thompson Ramo Wooldridge, a Texas company. The patent has expired.)

But there is more urgent than software-driven transmissions in the news. Toyota is hit (or has hit itself) by one problem after another. After recalling something like 8 million cars because of a combination of pesky floor mats and sticky gas pedals, we now hear Toyota’s crown jewel, the lated model Prius has buggy brakes.
How come?
Software, of course. And the need to score high numbers in the mileage tests.

Here is how the problem builds.

First, brakes.

Once upon a time, brakes were simple: you pushed on the pedal, the pressure was hydraulically transmitted to the brake itself. Assuming a disc brake, the pressure caused calipers to squeeze the disc between two sets of pads, thus slowing the car. For convenience and safety, your foot’s pressure is multiplied by a brake booster.
So far, no microprocessor.

Then we invented anti-locking brakes, ABS. As you know, when wheels lock, the car skids, braking is impaired, stopping distance increases. ABS uses sensors to monitor wheel motion. If the wheel abruptly stops moving, it’s locking. A software program then directs the brakes to lower the pressure on the disc pads, the wheel starts moving again, until it locks again and the unlocking cycle restarts. You can sometimes sense a vibration when the ABS quickly repeats the sequence, trying to keep the car just at the edge of maximum braking without locking the wheels. Nice. Modern implementations have faster actuators, the device that temporarily reduces hydraulic pressure, and smarter software, to better deal with road surfaces with gravel or snow where some amount of wheel lock is the better strategy. There is even newer code that detects a panic stop and forces the kind of maximum pressure normal drivers are reluctant or unable to apply.

Now on hybrids.

These cars reduce gasoline consumption by storing and re-using the otherwise wasted kinetic energy dissipated when the car slows down. Kinetic energy is a fancy name for the energy you must spend to give the car its movement. That energy manifests itself when you use the brakes: the car slows down, the kinetic energy is now heat in the brakes.

Let’s do some numbers.

At 100 km/h (62 mph, 27.8 m/sec), the kinetic energy of a 1,379 kg (3,042 lb) late model Toyota Prius is approximately 520 kilojoules. See a kinetic energy calculator here. If it takes 3 seconds to bring the car to a complete stop, on average, the power dissipation will be 520 kilojoules divided by 3 seconds, 173 kilowatts! (A watt is a joule per second). That’s a huge amount of “instant” power. For scale, the Prius features a 73 kW gasoline engine and needs to use it for a little less than 10 seconds to bring it to 100 km/h.

To recover and store kinetic energy, hybrids use a motor/generator: you give it electric power, it produces motion, you apply motion to it, it produces electricity. In other words, recovering kinetic energy uses something similar to engine braking on a conventional gasoline engine.
So, to slow the Prius, the car’s computer engages a clutch, the motor/generator is connected to the wheels and, for three seconds, converts the car’s motion to 173 kW of electric power to be stored in the hybrid’s battery for future use.

Not quite.

For the storage of recovered kinetic energy, the Prius uses a 201.6 volts battery (168 NiMH cells at 1.2 volts each). Divide 173 kW into 201.6 volts, to get the current intensity, the amperes used to transfer the electric power to the battery. You get about 806 amps! No battery can withstand such a charging current. The 6.5 amp-hours NiMH Prius battery shouldn’t be submitted to more than a few amps of charging current, not hundreds.

This leaves us with two solutions: lengthen the slowing down process and/or letting some of the power/energy go to waste.
But you can’t take 60 seconds or more to slow the car just to get a more manageable number of amps produced by the electrical engine braking process.
In real life, we need to stop when we need to stop, not when the computer feels it’s better for the battery. That’s where the computer, the barking and energy recovery software come in. When I hit the brake pedal, it does its best to blend “wasteful” disc braking and “good” regenerative braking, all as invisibly as possible.

To summarize: braking isn’t braking anymore, especially in a hybrid. Hitting the brake pedal triggers software processes that involve optimizing kinetic energy recovery while maintaining safety and comfort in a wide range of circumstances, including slippery roads and panic braking.
The the latest Prius, the so-called ZVW30 model, a.k.a 3rd generation Prius launched in 2009 is slightly heavier, 62 additional kilograms, and sports a beefier engine, 1.8 liter vs 1.5 liter displacement for the 2nd generation. Yet, it claims better EPA combined mileage, 50 mpg vs. 46 mpg.
Take a guess. How did that happen? How does a slightly heavier car, with a bigger engine and no appreciable aerodynamic or rolling resistance improvement over the very optimized  2004-2009 model get almost 10% better mileage?
The brake system and engine braking software has been “improved”, yielding better kinetic energy recovery by sacrificing some “classical” braking performance.
Unfortunately, it looks the optimization has been a little too aggressive, yielding brake performance trouble.

Speculation on my part?


But take a look at this Autoblog report of braking trouble with a Ford hybrid:

As one of our senior engineers slowed for a stop sign at the turnoff to our test facility in East Haddam, Connecticut, the brake pedal went unexpectedly further down than normal but the car barely slowed. He zoomed through the turn, with brake-system warning lights illuminated on the dash. The car more or less coasted to a stop, with what our engineer described as minimal brake feel.

Ford announces they’ll fix the software on their hybrids’ braking systems.

Toyota had discreetly updated the software on “factory-fresh” Prius, they’ll do the same, one hopes, for their customers who bought the 3rd generation Prius thinking they were safe with the Japanese automaker.


Print Friendly