This is my first post on this forum. I enjoy reading the opinions of car enthusiast.
I have noticed that on the highway my engine (HIHY) will shut down during periods of coasting even at 65 mph. This happens mostly going down long hills. I have heard that engines are more efficient at higher HP output. Do you think Toyota is charging the battery during the up hill high HP sections of the road in order to run on battery power during the down hill sections? I would have guessed that it would be the oppisite. Use the battery to help the vehicle up the hill and charge on the way down to keep the HP output lower and more constant.
If engines are more efficient at high power like a deasel for instance doesn't it make more FE sence to accelerate hard from the light and coast as far as possable to the next stop?
Chuck

 

In certain situations doing that (charging during heavy load, engine off costing the rest of the time) may be better off. One case is where you only need a tiny amount of horsepower, but you are travelling at high speed. Turning the engine at 60mph at such a low output, you would be wasting most of your gas just to overcome engine braking. Charging while the car is moving may seem somewhat counter intuitive, but most of the time (speaking from experience in my Honda Civic), this is done when the engine is running below its peak efficient output, or during flat cruising.

This is the main idea of the hybrids -- it also allows them to produce the same power with a smaller engine. On cars with oversized engines, lots of gas is wasted pumping air through all that extra cylinder displacement -- with a smaller engine and a motor, you can get the same peak power when needed, without the parasitic waste.

This does make sense. In most cases, when cruising at 70mph the engine output will be more efficient, but that improvement is far outpaced by increasing air resistance, so total gas mileage is much worse than say 40mph. However, in the case of acceleration, it takes a certain amount of kinetic energy to reach a certain speed no matter what, so it's best to run with the engine at its most efficient output when accelerating -- usually a moderate acceleration.

Engines are not necessarily more efficient at high power, as peak horsepower requires revving up to high RPMs and gearing the output down -- this leads to a lot more friction/geartrain loss. Also, at max power output, the fuel mixture does not combust as thoroughly. Engines will tend to be most efficient when running with a high torque output, in the tallest gear ratio (lowest RPM) possible.

In my Civic the engine runs all the time -- the point where the electric motor just starts to kick in to assist the engine appears to be its most efficient output. It doesn't make sense for the motor to assist unless the engine has just reached is most efficient point, which appears to be about 1/2 load at around 2000 rpm.

In the case of the Toyota designs, they will tend to run with the motor alone at low output, then kick in with the engine alone, later, at the point where you first start to see the engine and motor contributing at the same time will likely be your most efficient throttle position under acceleration.

 

Thanks for that reply. The led foots will be pleased to know that they might not be wasting as much fuel as they thought.

In the case of the Highlander it has close to 200kwh of power and if the meter on my dash can be believed it takes only about 70 - 80 kwh to maintain 65 mph on a fairly descent uphill grade. That might be in the efficiency range to add battery charging.