How can two engines have similar power & torque but hugely different fuel consumption? Are bigger (capacity) engines intrinsically thirstier? Details next.
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Here’s a question from a dude named Dave, concerning the King (of mediocrity) - Toyota.
"I'm comparing a Landcruiser Troopie with a new Prado. The Troopie has a V8 with 151kW & 430Nm. The Prado has an inline four-cylinder with 150kW & 500Nm. (Approximately the same power/torque.) But the Troopy uses about 15L/100 compared to the Prado at 10L/100 (both ballpark real world figures, not the ADR fantasies).
"If they output about the same power, where is the big difference in fuel economy coming from? The weight of each car? Aerodynamics? Gearing? Feeding four extra cylinders, with extra engine internal friction?
"If they were the same weight, and had the same Cd value, would they accelerate/overtake the same and have the same fuel usage, even though one is V8 and one an I4?" - David Deere
The data:
Diesel = 45 kJ/g (and if you do that once a second = 45kW)
So, 2.2g/sec = 100kW (of potential heat energy)
Combustion efficiency = 40% = 40kW @ crankshaft
15% loss via transmission = 34kW @ wheels
Prado:
2.755cc
6sp (5th = 0.687:1, 6th = 0.580:1, diff = 3.91:1)
265/65R17 (Circ. = 2.26m)
150kW @ 3400rpm
500Nm @ 1600-2800rpm
Bore x stroke: 92.0 x 103.6mm
ADR economy: 7.9 (combined, L/100km)
Kerb weight: 2325 (GXL)
100km/h = 27.8m/s
= 737 wheel rpm
= 2883 tailshaft rpm
= 1672 engine rpm (6th)
= 1981 engine rpm (5th)
Total swept cylinder area = 120,000 mm^2
Troopie:
5sp (5th = 0.75:1, diff = 3.91:1)
225/95R16 (Circumf = 2.62m)
4.461cc (+62% move volume)
151 @ 3400rpm
430Nm @ 1200-3200rpm
Bore x stroke: 86.0 x 96.0mm
ADR economy: 10.7 (L/100km) - about 35% higher than Prado
Kerb weight 2265kg
100km/h = 27.8m/s
= 636 wheel rpm
= 2487 tailshaft rpm
= 1865 engine rpm (5th) = 11.5% higher than Prado in 6th
Total swept cylinder area = 207,000 mm^2
= 72.5% more area than Prado
** but at 100km/h rpms are also 11.5% higher, so total cylinder wall friction = 81% higher (ballpark)
** Hyundai/Kia 2.2 @ 147 kW & 440 Nm I4
** Mazda 2.2 @ 140 kW & 450 Nm I4
Plus: Troopy needs to drive twice as many valves to do the same job, pumping more oil and water, etc. (Not as volumetrically efficient, and higher pumping losses, etc.)
And the single turbo 4.5 V8 is a rubbish engine - like it’s the cheap, slutty version of the 4.5 TT, which makes 32.5% more peak power, 51% more peak torque and returns slightly better fuel economy - despite being in a platform weighing 500kg more.
