Clett wrote: "For example, look at how much power it takes to haul a 40-tonne load up an average 1:30 gradient at 60 mph: That's 0.87 ms-1 vertical, Power to overcome gravity only = mgv = 40,000 x 9.8 x 0.87 = 341 kW. This is why HGV engines are grossly oversized compared to the amount of power required to propel them at 60 mph on-the-flat."

Clett, a "grossly-oversized" engine wastes fuel at cruise and adds unnecessary weight to the truck that will take away cargo load capacity, as well as higher purchasing cost hence higher amortization cost.

Since it takes 200 kW or 268 hp to cruise at 60 mph out of 450 hp maximum, there is still power left to accelerate and to maintain speed in a mild upslope. For higher grades, the truck simple slows down a bit as it reaches to the peak, and then accelerates downslope back to its regular cruise speed. This has been my observation while driving in the highway. What's your observation, Clett?

Slowing down going uphill and speeding back up going downhill is the simplest way to maintain the engine running at its maximum thermal efficiency WITHOUT the considerable additional expense and weight of a hybrid drive train. For longer slopes after the truck has slowed down enough, the truck will be downshifted to lower gears in order to maintain a steady but slower climb-cruise speed.

I don't agree. The power required to accelerate and climb grades are vastly more than that required to maintain 60 mph on the flat. 100 kW for 60 mph on the flat, 441 kW to maintain 60 mph up a 1:30 grade.

Look at how much power is required to accelerate too. Imagine accelerating from 0-60 mph over 60 seconds. Kinetic energy of a 40-tonne HGV at 60 mph = 0.5 x m x v x v = 13,520 kJ. Power required to do that over 60 seconds therefore = 225 kW - not including the base power for overcoming resistance!

Climbing and accelerating require vastly more power than cruising on the flat, and that's why the engines are rated at much higher power than used at cruising.

If this wasn't the case, and we use your assumption that HGVs use 200 kW at 60 mph cruise, there is only 100 kW in reserve. That would mean HGVs would only barely be able to maintain 60 mph on a 1:100 incline! In reality, this is not the case.

Clett,
There is no need to maintain 60mph when climbing any grade. There is no code in the book that specifies that truck must maintain 60 mph when climbing uphill.
I often drove behind these big trucks in the past,because they have radar detector and I don't, and the experience has been that I can keep my foot on the same position on the gas pedal regardless whether going up or downhill. My car slowed down a bit going up hill, thus avoid overstressing the engine (old NA carburetted V-8 knocks when you push on the gas pedal going uphill), and speeded back up when going downhill.

Now, considering this: BSFC for a typical truck diesel engine is 0.33 lbs/hp/hr. Just look this up any reference book! At 100 kW, or 134 hp/hr, the truck will consume 44 lbs of fuel. Way too low!

Thsese same trucks are reported to go 6 mi/ImpGal by US truckers. 60/6 = 10 ImgGal/hr, and at 8 lbs/ImpGal, the same truck consumes 80 lbs of fuel. Now, this the reality, Clett, accept it!

If your method of calculation was correct, then HGVs would not be able to climb hills! Let's say they do dip down to 55 mph on a 1:30 incline and use 180 kW. There's only 120 kW spare, which is only enough to overcome the gravity component alone at 23 mph (120000/9.8/40000/0.03 ms-1)!

Here in Scotland we have many long sections of road at 1:30 and the HGVs only drop down to at least 55 mph on the steepest sections by the top. Hills have little effect on cars, where the mass component is not great, but a very large effect on heavy goods vehicles.

It's clear that we aren't going to agree on this one so perhaps the best thing to do is wait until the BE-HGVs come on the market. I'm sure you think they never will, but my view is that the cost per electric mile (here in the UK) would be only 5 pence per mile for electric use, but 64 p per mile for diesel use. With the operator saving about £80,000 per year in fuel costs, BE-HGVs will make sense very soon. With a break-even time of 6 years, the 1 MWh battery would have to cost less than £480,000, or about £480 per kWh. Today's 18650 lithium-ions cost around £200 per kWh to manufacture, so how long can it be before large format 330 Wh/kg Electrovaya packs also hit this target?

Clett,
Not all the trucks you observed were loaded to 40 tonnes (88,000 lbs). More likely, most of them are loaded to 60,000 lbs, or ~2/3rd of maximum. Let's re-do the calculation with a GWR of 60,000 lbs instead:

At that weight, it will take only ~135 kW to maintain cruise at 55 mph instead of 180 kW, so, from 335 kW (450 hp) total power, you will have 200 kW of power to spare.
So, 23 mph /60,000 x 88,000 /120 kW x 200 kW = 56 mph! Voila, most trucks at 60,000 lbs can maintain 56 mph just fine for a 1:30 grade.