Assuming everything else is equal (vehicle size, weight, driving style, tires), how do you make ICE engines be more fuel efficient? I noticed my dad’s 1996 Nissan Pathfinder V6 consumes 13L/100km (mixed driving) while a newer V6 is 9L/100km. Car manufacturers claim their engines are “more fuel efficient”, how do they accomplish that?
In: 85
Ok, so there is a pretty big list of things that go into this. Here is what comes to mind.
**1: Things that let the engine operate in a better speed/rpm range.**
In general you want your engine to be spending most of it’s time at low-mid rpm (say 2000 – 3000) and close to full throttle. Lower RPM = less friction losses, higher throttle = less work spent pulling air past a closed thottle. See this graph for [torque and rpm vs efficiency.](https://www.researchgate.net/publication/316530475/figure/fig4/AS:495842618089478@1495229500689/BSFC-map-of-the-engine-7-BSFC-brake-specific-fuel-consumption-rpm-r-min-Max-maximum.png)
Of course making more power (via higher throttle for example) will use more fuel in total, but for each HP generated you’ll use less fuel.
So, if you make the engine as small as you can, then you will use a higher percentage of maximum torque/throttle at cruising speeds, and you’ll be running the engine in a more efficient area of that load graph. (near the top) As long as you don’t make it so small that you need too many revs to make enough power (too far to the right on the graph)
More gears also help you be able to be in that best efficiency area of the graph more of the time, and CVTs are even better.
Turbos don’t help fuel economy as such, but they DO let you make an engine that is way too small for good acceleration, run it at high load during cruise, and then when you need power you just throw boost at it. (see also downsized things like the 1.0 ecoboost). This only gives good economy if you don’t use the power much though.
**2: Things that extract more energy from the same amount of fuel.**
Higher compression ratios get more energy from the same amount of fuel. Both by squashing the air fuel mixture more (compression ratio) and by expanding it more when it goes bang (expansion ratio)
If you can design a better combustion chamber you can get more complete combustion of the fuel too and by avoiding knock you can run more aggressive ignition timing, which again lets you extract more power from the same fuel by firing the spark at a time that imparts power more effectively to the piston.
Direct injection as well as knock sensing both help get you closer to that knock limit, running higher compression and more aggressive timing.
**3. Things that control Air Fuel Ratio better.**
Fuel injection vs carburettors basically. The better control you have over air fuel ratios, the more you can get the engine to target the most effecient ratios as often as possible.
**4. Valvetrain trickery**
Cam profiles are optimised for different engine operating speeds and loads, so a cam profile that’s best for low load, low RPM cruising, will be bad for power at high load and RPM.
Want the best of both worlds? Variable valve timing (and ideally lift) lets you make the engine happier at every combination of load and RPM.
Remember how the reason to run a smaller engine was that you could use higher throttle more of the time, and reduce pumping losses? We can cheat this with valve timing too, this is how Atkinson cycle works.
Example (the percentages are wrong, but gives you the idea): Lets say for (crude) argument’s sake we need to fill the cylinder 40% to make the power we need to cruise. We could run at 40% throttle, and waste a lot of energy pulling air past a closed throttle. OR… we could run at 80% throttle, fill the cylinder 80%, but then leave the intake valve open during the first bit of compression to push half of the air back out again. We make the same power, but with way more throttle and less pumping losses.
The other win is we can use this to gain more expansion ratio. Expansion ratio raises exactly the same way compression ratio does. Problem is we can’t raise the compression ratio too far without knock. But what if we still want to raise the expansion ratio more?
Just raise the compression to like 15:1, but then to prevent knock we leave the intake valve open during a bit of the compression stroke. This lowers the compression ratio, because it’s pushing air out again instead of compressing. BUT we still get a higher expansion ratio, and get more power from the same air and fuel. Again this is the Atkinson cycle trick.
**5. Things that make the engine easier to turn**
Low friction coatings inside the engine, softer valve springs, thinner oils, electrically driven accessories like water pumps, alternators that only run hard on deceleration (when you already want to slow down)
**6. Things that make it easier to move the car**
Aerodynamics, weight reduction, more efficient gearboxes with less friction losses, more efficient tyres with less rolling resistance. Less power to move car = less fuel used.
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