Brakes on electric cars

[DJP]

I was following a Nissan Leaf yesterday, which had a sticker proudly proclaiming itself to be “100% Electric” and I got to thinking:

If there's no engine to provide vacuum to the servo, how do the brakes work on electric cars?

Or, likewise, on a hybrid when it's running in electric mode?

Or do they have a completely different system?

[kambites]

They run an electric vacuum pump, I believe.

[graham22]

I'm guessing turbo charged cars don't have a vacuum in the intake manifold either?? How do their servos operate?

[snotrag]

graham22 said:

I'm guessing turbo charged cars don't have a vacuum in the intake manifold either?? How do their servos operate?

As soon as you take your foot off the throttle, you have vacuum. Brakes work just as normal!

[The Wookie]

I think the leaf has some weird setup with an electric motor altering the pedal stroke and a hydraulic accumulator to provide the servo assistance.

On a project at my previous job where we converted a normal car to be a SHEV, we used an electric vacuum pump.

Also worth pointing out that some cars do actually have a mechanically driven vacuum pump off the engine

[Major Fallout]

My 1998 range rover has an electric pump for the servo.

God knows why you would trust lucas with stopping a car!

[996TT02]

Nothing new there, diesel cars don't have a throttle flap except for engine shutoff so no usable vacuum either, most have a separate vacuum pump coming off a drive belt.

In case you were wondering, throttle is controlled by fuel feed i.e. off the HP fuel (injector) pump.

This is OK because diesels are OK at running extremely lean.

[anonymous-user]

As most cars now have some form of stability control which requires active intervention from the brakes even when the driver is not pressing the brake pedal, they have enough hydraulic power to negate the use of a brake servo. So, cars designed as EVs/Hybrids from the ground up use a fully hydraulic pressure booster / blender, whereas converted ICE cars usually just end up with 'lecy vacuum pumps for a conventional servo. The really tricky bit is getting a smooth blend between regenerative brake torque and mechanical friction brake torque, as the co-efficient of friction of a conventional brake system is very variable!

[alangla]

Max_Torque said:

As most cars now have some form of stability control which requires active intervention from the brakes even when the driver is not pressing the brake pedal, they have enough hydraulic power to negate the use of a brake servo. So, cars designed as EVs/Hybrids from the ground up use a fully hydraulic pressure booster / blender, whereas converted ICE cars usually just end up with 'lecy vacuum pumps for a conventional servo. The really tricky bit is getting a smooth blend between regenerative brake torque and mechanical friction brake torque, as the co-efficient of friction of a conventional brake system is very variable!

I'm still waiting for the EV that only uses the friction brakes for emergency/parking purposes. A lot of other electric vehicles (e.g. trains/tube/trams) do all or nearly all their braking either regeneratively or rheostatically, meaning no brake pad wear at all. The only problem I guess is getting rid of the heat generated by a rheo brake (then again, you can use some of the electricity generated to turn a fan blowing over the resistor grids!)

[saaby93]

Well you dont need a servo there are cars with servo less brakes
In the Prius thread the discs and pads last far longer than a normal car if you stick to braking gently using regen.