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What is driving the pace of change in brake systems?
One of the major drivers is meeting the requirements in the Transportation Dept.’s New Car Assessment Program for automotive emergency brakes (AEB). Other key drivers include the demands for driver-assist systems and higher performance brakes.
“Driverless” cars are also pushing the need for autonomous braking. These will range from electronically controlled deceleration for adaptive cruise control up to AEB and electric parking brakes with auto hold.
Another driver is the need to meet emission-reduction targets. It is affecting which fuel-efficient technologies and systems are adopted that allow recuperation of energy during braking. Development work at auto companies is also focused on finding ways to reduce residual drag due to brakes and work with start-stop function. (Start-stop shuts down the car’s internal combustion engine when the car isn’t moving to reduce the amount of idling time to save fuel and reduce emissions.) It’s estimated that across the globe 30 to 40% of new cars carry start-stop, and this will soon rise to 50%
With brakes, every automaker wants the same thing: smaller, lighter, faster, safer, and more durable. Of course, they also need to become more and more affordable year after year.
What effect is automatic emergency braking having on your work?
AEB is driving us like nothing else. We have worked hard to understand what the brakes and sensors can deliver in terms of stopping time and object recognition. We’re talking with customers about improved braking systems that can automatically bring a car to stop in under 300-400 milliseconds.
This is important because the brake’s response speed determines how much time the sensors have to analyze a potential emergency situation. Today’s stability controls systems are quick, but not quick enough to decelerate a mid-sized vehicle at 1 g. Everything has to optimized, from the brake control unit to the calipers to the system’s fluid dynamics. We have clear development plans to achieve very fast pressure-apply with conventional electronic stability control (ESC).
How is TRW improving regenerative braking?
It has developed a premium hybrid system, ESC-PH, based on a six-piston pump which can rapidly deliver high levels of brake pressure and regenerative braking. It also is equipped with an innovative valve-control algorithm that lets the system convert more than 90% of the brake energy into electricity at certain speeds.
What effect does start-stop have on braking systems?
If you have driven a car with start-stop, you know that when you stop at a light, sometime the engine shuts down, sometimes it doesn’t. That’s because the decision to switch off the engine is safety related. The car needs to know it has the power to stop and restart safely. For that, there needs to be enough pressure in the brake system, enough vacuum in the brake booster, and enough electrical power onboard.
So it’s important we ensure that the hydraulic hold and electrical parking brake carefully cooperate—and in a way that doesn’t alarm or confuse drivers.
How can brake systems make start-stop more beneficial?
When cars stop, drivers need to rest their feet on the brake pedal. To make this comfortable and safe, the brake booster must maintain its vacuum. The longer the vacuum lasts, the longer the pump and engine can remain off. And the longer they remain off translates into fuel and CO2 savings.
To make start-stop systems even more efficient, we need better sealing for brake boosters to hold the vacuum longer. In the past, leaks were acceptable, but now tighter boosters are the key. TRW, which makes vacuum sensors, is working on new ways to add them to the system and place them close to the booster.
Why is reducing residual brake drag so important?
If we can reduce residual drag by just 0.5Nm per caliper—2 Nm for the entire vehicle—you can save up to 0.1 liters of fuel per 100 km and eliminate 1g/km of CO2. That might not sound like much, but in the future, the penalty for emitting a single gram of CO2 will be hundreds of Euros.
Residual drag is an important factor in maintaining pedal feel, stability, and wet-braking performance. So we have to try and reach zero-drag without compromising these and other braking functions.
How much potential is there for even greater integration in brake systems?
TRW’s Integrated Brake Control (IBC) remains a key advanced technology in brake system integration. It replaces the ESC system, vacuum booster, pumps, and cabling with a single unit. It is lightweight and the pressure dynamics are perfect for AEC, making its suitability for driver assist functions compelling. IBC is also ideally suited for regenerative braking and full brake blending.
We now have a core architecture that lets us re-use components on braking systems for follow-on cars and platforms. The technology can be scaled to be suitable for everything from entry-level vehicles to high-end requirements.
Manfred Meyer is vice president, braking engineering for TRW Automotive, Cleveland, Ohio.