Recently started a new commute which involves descending a hill for 2-3km(elevation change around 150 meters) right after startup at speed limit of 50kmh before merging onto the highway(80kmh). the grade is just enough that i can ride down in b mode and only needing a very light pressure on the brake to control speed. Been experimenting and noticed that b mode will indicate zero fuel consumption while descending the hill whereas leaving it in drive and braking on my way down will indicate fuel being consumed to idle the ICE. My question, is it more effecient to use b mode in the beginning of the warmup stage versus drive mode? I know not everyone has a nice easy hill to descend at the beginning of their commute to warm the ICE but if you did, is this what you would do? seems most economical to me, as the ICE needs to warmup anyway, so why not shut off fuel as b mode does and use that wasted heat energy to speed the warmup process. Or does it in fact warm up faster if left in drive mode?
While I agree with Mr grimes, warm up is one of the most wasteful times for the engine. At least some of the 'wasted energy' should be warming the ICE, speeding up the period of poor economy from the ICE. While it would never occur to me to use B unless I was on a long hill, I would believe an instrumented test that stated that Mr b's technique is better in his particular commute.
While you're consuming some fuel during warmup you're also adding energy to the battery. If, instead, you stay in D for the descent controlling your speed with the brake pedal you should be adding even more to the battery. If you're back-driving the engine in B to the extent that it isn't consuming fuel that means that it also isn't proceeding through the warmup cycle. He last thing you want to do is delay the warmup stage (1a) till you actually need to propel the vehicle. That's my take on it.
I'll test out D and B next week and see what difference it makes. Just an additional note, the hill is long enough that i can usually descend in B mode and still have a full battery at the bottom, so i'm not missing out on charging the battery by doing this. By running in D mode i just end up heating the brake rotors to descend the last bit of the hill. I guess what i'm curious about is if running in B mode actually warms the ICE faster or slower than just idling, burning fuel as it does in D mode, or does it in fact "interrupt" the warmup cycle as css28 says? The warmup time on this section of road makes a bit of difference because at the bottom of that hill i then have to accelerate up to highway speed(90kmh) so having a warmer ICE at this point would be advantageous to get the best Highway fuel economy.
You can't warm the engine unless you burn some fuel (except some minor heat generation due to friction). But, it may be advantages to delay running the engine to a time that you need the ICE anyway to provide power.
I'll wager that with today's lubrication technology and engine tolerances, you could turn that engine all day with no fuel and generate very little heat. Any heat gained in B mode would come from air compression, and not very much at that. I think the objective should be to arrive at the freeway on-ramp with a warm engine regardless of whether you burn a extra bit of gas to do it. So its not about efficiency going down the hill, its about efficiency on the freeway. For fun it might be interesting to use the cruise control system going down that hill and see how it handles this. Just be aware that if the battery fills, the CC system will start using only engine braking and you may accelerate.
Well, that actually makes perfect sense now that I think about it some more. I guess I was over thinking the whole thing. Warming your hands by a fire is certainly easier and warms them faster than rubbing them together.
I remember someone here giving the figure of 1kW for turning the engine (in highway stealth mode). That would be 3413 BTU/hour. Gasoline (at 50 MPG, and 50 MPH) would be 110,000 BTU/hour. Compressing air could be quite a bit, since it needs to be the equivalent of moderate braking. 150 meters drop would be around 2000 BTU for the weight of the V.
We should keep in mind the "engine" warm-up is a way of preheat catalyst to improve emissions performance. That mission may be the reason for the procedure.
I've been avoiding b mode since receiving feedback in this thread. i think its making a bit of a difference, though not measurable by avg tank mpg so far. seems i'm underestimating the regen capability. The HSD seems to do a pretty fine job of capturing as much energy as possible through regen and adding friction braking as needed to slow the car just keeping it in normal drive mode.
