Cruise control question.

Also, if you have a gentle foot, you can just use the brakes but press gently so you never go beyond regen for braking power.

 

Question, is there any harm in holding down the cruise control lever (deceleration) for slowing the car & recharging the battery ?


Rob43

 

I'm not certain, but I don't believe your brake lights will come on if you do that. So yes, if you consider getting rear-ended to be "harm".

What's wrong with using the brake pedal? It's designed exactly for slowing the car and recharging the battery.

 

I don't really care about the brake lights coming on because I only do this when there's no traffic behind me.

So my question still remains: Will any harm come to the car if I slow down & recharge using this method ?


Rob43

 

No

 

I'm surprised that no one has mentioned the engine braking that occurs once the battery is full and the cruise is still trying to keep the car from picking up speed while going down a long steep hill. When it first happened to me, I didn't like the thought of it may be using fuel during this but if the trade off is more wear on my pads and rotors to maintain a safe speed, then I'll take a minimal fuel consumption trade off any day. Makes me wonder about a full EV in the same scenario since they don't have an ICE to use compression to help slow the car. Maybe an EV owner or anyone with EV experience can chime in.

 

No, not as far as any of us know. In fact, I'm trying to think if there's *any* way you can really damage the car by driving it 'wrong'; it's pretty idiot-proof. Trying to put it in park while moving will go to neutral instead, if you're above about 10 mph or something like that. (Granted, 10 mph to park is apparently rather jarring...) Similarly, I believe shifting to reverse while moving at appreciable speed does the same. If you hit the brakes and gas, the brakes win. The redline speed is enforced by software. I suppose you could leave the parking brake on... Or shift to neutral at 45 mph down a very steep hill and overspeed the electric motor - although I'm not certain that you'd be able to immediately cause any permanent damage that way.

But I still don't see the point. Using the brake pedal will do the exact same thing, and recapture just as much energy (as long as you keep it within the CHG bar in the HSI). But go for it.

 

We know from Toyota literature that when you take your foot off the accelerator the "drag" you experience is the HSD "simulating" engine braking in a "normal" car. It's a pretty small amount of drag. It's needed because the Prius HSD has very low resistance - the car will not slow down much unless some form of drag is added.

In cruise, a lot more drag is used in an attempt to maintain the set speed. Keep in mind, -some- police (I shall refrain from calling them names ) like to set up their speed traps on downhills, because they know many people don't pay attention and tend to speed on them. So this cruise control extra drag from regen. is "a good thing" (tm). Be warned, on very steep downhills it is not enough!

I too have wondered if the brake lights are used in cruise regen. braking, and I concur probably not.

DO keep in mind, the GIII cruise control use of regen. braking can result in putting you in dangerous situations. When I picked up Pearl S and was driving her for the first time, I was using cruise control along a secondary highway. Approaching a small town the speed dropped from 100 km/hr to 70, so I just dropped the set speed using "down, down, down, etc" on the cruise lever, something I always used on Pearl (GII). Good thing the pickup driver behind me was attentive! Pearl S braked quite quickly to get to 70. So now I just cancel cruise, and when Pearl S coasts down to the correct speed, reset it.

 

I do the same. A common location is the east end of the Barnett Highway, coming into Port Moody: speed goes from 80 (kmh) to 60. Pull the cruise stalk towards me (cancelling) at the "60 kmh ahead" sign, tap down (setting) when speed falls off to 60.

 

Can you confirm: if you're for example going down a hill with cruise on, and gravity/inertia is accelerating the car, and regen activates, over some threshold it'll turn on the brake lights? That'd be good. You might even be able to check this out with some sorta reflector on the high mount brake light. Or a volt meter with long leads.

 

I have to drive through three of these "traps" every day on my commute. I can confirm that with cruise control set there is extra drag. If I release cruise at the top of one of these hills the car will maintain speed and depending on wind speed and direction might accelerate 1-2 mph. With cruise on it decelerates about 5 mph from the top of the hill to the bottom. I would estimate maybe a 4-5% grade on that hill.

 

It's a highly annoying and wasteful bad thing when I'm using cruise control to maintain a reasonable speed on an Interstate, and it brakes needlessly on moderate downhills that I could safely and legally coast down a little faster without the automated meddling. I'm capable of deciding for myself when using the brake or "B" is appropriate .

That unwanted braking can be avoided by feathering the accelerator while watching the HSI bar, but it's a lot more touchy (even in ECO mode) than when cruise control is not on. Or cancel cruise, and then be careful to resume only when the car coasts down very near the previously set speed, because if you're a little too early or too late (i.e., speed is faster or slower than set) it will brake or accelerate to the set speed needlessly abruptly. Overall, the thing tries to hold the speed too rigidly, which wastes fuel in rolling terrain.

 

Most vehicles with automatic transmissions DO NOT get any amount of effective "engine braking" in D mode because a torque converter isn't as much of a "direct connection" between engine and transmission as a clutch plate. So, they have to go into 1 or 2 to get that.

Normally, the trick is to apply the brakes to scrub off speed in a straight line at intervals so the pads/shoes have time to cool. Most downhill runs are designed to prevent people "cooking" their brakes. Vehicles hauling loads are more at risk because of how much inertia they build up in a short time. The times I've seen downhill runs long enough to "cook" brakes are often mountain roads where it's constant downhill and poorly planned road design.

 

I would much rather "use" my friction brakes that are significantly underused rather than use the engine compression for braking. The brakes are a far simpler/easier/cheaper repair scenario than the engine.

Curiously; before I knew better I was coasting downhill a long grade in neutral in excess of 70mph & my car works normally 60,000 miles later. I'm assuming the software actually had the car in D regardless of what I was asking of it to protect MG1.

My understanding is that modern day A/T's with locking torque converters do in fact allow & use engine brake since they are literally mechanically coupled to the engine during lock-up. Most A/T's will lock up mechanically as low as 2nd or 3rd gear for better fuel economy. That's how A/T's started achieving higher highway MPG's. A numerically lower gear ratio for lower engine RPM's at highway speeds while mechanically locked compared to equivalent manual transmissions with the ability to unlock for higher RPM's under load driving up smaller hills for example.

 

What's seriously psychic: when I think of testing this out, I was picturing east Hastings, eastbound, coming down the hill approaching shell refinery, pretty close to what you mentioned. And I figured you were on the prairies somewhere.

 

I learn something new every other day.

 

I have noticed when I apply the brake over a road bump or on a rough road the brake skip or will not hold with the same force as on a smooth road . Has any one experienced this ?