Is there any logical reason why the ICE needs to run when travelling on the highway at highway speeds? This may have been covered before, but I couldn't produce anything while searching.

On the highway portion of my normal commute there are several large downhill sections, some approaching a mile in length or maybe even more, where the vehicle can literally just coast down without losing speed. Why does the ICE need to run here? I'm just envisioning the amount of extra fuel being burned for no logical reason at all.

Or am I just overthinking the whole thing....?

I'm not sure about the MMH but there is a way to trick the Prius (which uses a similar HSD system) into not using any gas on downhills. It may even do it automatically in fuel cut mode? I'll try to find you a link.......

Warp Stealth (Prius)

When driving at speeds faster than 42 miles per hour (68 km/h) with excess momentum propelling the car (like when you gain speed going down a hill), the engine will continue to pump the pistons; however, fuel won't always be supplied. This avoided waste increases overall MPG. Owners have dubbed this condition: "Warp Stealth"

The power split device requires the engine to be running to prevent overspeeding the motor-generator. The max speed on the MG is 10,000 RPM which equates to ~40MPH. Anything over that requires the engine to spin, although, as lakedude pointed out, there are times when there is no fuel to the cylinders so it is a very smart vehicle in that aspect.

My 4WD
Wife's 4WD

Thanks for the replies, curiosity got the better of me....

The GS450h hybrid system has a two speed + CVT design to overcome some of the limitations of the generator speeds. The engine can shut off at higher vehicle speeds due to this design - it uses one ratio at lower speeds and the second at higher vehicle speeds.

Thanks, I was wondering the same thing as Green Monster. On my first couple of freeway drives, as well as my daily commute, I have times that I'm above 50mph and the engine is still on. I figured I'm saving some gas since the rpm is only 1000 instead of the usual 2000-4000. But, if I can gain speed coasting, why can't the engine stop?

BTW - I've actually been able to coast on EV up to 35mph, and even climb a couple of mild hills at 30mph. This is much better than Ford proclaims.

30+mpg vs. 20+ mpg with a minimal decrease in cargo-room -- why should someone consider a nonhybrid?