Engine MPG / Economy

Question

I drive a 4 cylinder Mitsubishi lancer evolution and I CAN get UP TO about 20 mpg on the highways.

What determines MPG? How is it that engines with twice the number of cylinders, and in some cases more power (HP/TQ) get BETTER gas mileage ?

Answer 1

Aerodynamic drag - The shape of your car has a direct effect on how hard it has to push through the air at a given speed. The squarer it is the more energy will be required to maintain your speed. The Lancer has large spoilers front and aft which create downforce, plus lots of cooling vents, at the cost of adding drag.

Tire friction - Fat tires made with a soft compound (higher grip) will have a higher rolling resistance than skinny tires with a harder compound. Wider tires have more wind resistance too. One is better when you want to be safe and the other is better for economical driving. The Lancer has sticky, wide high-performance tires.

Drivetrain friction - A 4WD car like the Lancer has a complex drivetrain with lots of rotating mass, gears and couplings. All of these have some rolling resistance/friction, so it's less efficient than a 2WD drivetrain.

Engine Efficiency - some engines are naturally more efficient than others. The 4G63 engine in the Lancer is an older design with much less regard to efficiency than more recent engines. Power saving items like electric power steering and other ancillaries, low-friction internals etc. were not used in the Evo.

Gearing - The Lancer has relatively short gearing with the goal of maximizing acceleration. This means the engine runs at relatively high rpm.

Vehicle Weight - Being a 4WD the car is relatively heavy.

Turbocharging (especially pertinent to an Evo) - When done for performance this usually ends up with the fuel mixture being run rich to prevent knocking. The engine will generate more power for its displacement but less efficiently. Turbo engines seem to be especially sensitive to the metal content of your foot.

Answer 2

What Determines MPG or miles per gallon?

This question is pretty wide and has many factors but i will highlight the most important ones.

• Heavy foot: A heavy foot is the prime suspect and usually the reason for poor fuel economy and plays a significant if not the primary role in determining the MPG your vehicle returns.
• Air-Conditioner/Electricals: Running the Air conditioner will massively tax your MPG especially in older vehicles , also using other electronic gadgets also further contribute to the loss.
• Wind resistance/aerodynamic drag: A boxy shape would obviously have more wind resistance thus reducing MPG.
• Load: The more weight you haul around the more work your engine has to do and thus bad MPG

The above 4 are the usual suspects in everyday driving which affect a good share of MPG, there are also other factors such as driving up a hill, driving too fast, engaging 4WD if your car has one.

Why do bigger engines give more MPG?

This is not essentially true at all, bigger engines do not necessarily give out better MPG.

A small 1.2 liter engine will almost always dish out much higher MPG figures than say a 5.2 liter v10 , the only situation where the V10 might give higher figures is if both the cars are running at say 110mph, in that scenario the 1.2 engine will be running at its limit and guzzling more fuel than the V10 for which 110mph is nothing.

However a common observation missed by many people is that a bigger engine(modern engine) gives out better "RELATIVE" MPG for example the 3.8 V8 gives out almost same MPG as your EVO that is due to them having bigger turbo, lower weight, better transmission, better aerodynamics, so its an illusion that bigger engines give better MPG. Although the truth is that they give Better Relative MPG, Relative to their Engine Sizes.

Answer 3

As a supplement to the other answers:

The ECU can be a factor as well. Some ECU's stay in open loop mode more than others. In general, if you push the accelerator pedal past a certain point the ECU will leave closed loop mode (where the air/gas ratio is controlled and can be 14/1 or greater in some cars), and go into to open mode where most ECUs use a 9/1 ration or so. (Not 100% sure about the ratio, but it is lower than in closed loop). This may also happen at high RPM, but I am not positive about that.

Additionally, the Mitsubishi Lancer Evolution has variable valve timing system, similar to Honda's i-VTEC, so the valve timing will be advanced and the valve lift can vary. So if you really want to maximize the MPG you will need to drive in such a way to prevent the ECU from shifting into the more aggressive valve timing and valve lift mode. On 2007 Honda Accord with i-VTEC this means keeping the RPMs below 2400. On the highway this equates to 65 MPH.

Also, the slope of the drag versus MPH curve increases with MPH. Here's some good information about the drag with respect to MPH:

https://www.wired.com/2011/04/what-will-an-85-mph-speed-limit-do-to-your-mileage/

The information can be found here for the newer model Lancer's:

https://en.wikipedia.org/wiki/Mitsubishi_4B1_engine

https://en.wikipedia.org/wiki/MIVEC

There's a good discussion of closed loop and open loop modes here:

https://www.ecmtuning.com/wiki/engineandecu101

Answer 4

I used to have a 1.33 litre 2011 Toyota Yaris with about 100 horsepower. Now I have a 2.5 litre 2016 Toyota RAV4 hybrid with about 200 horsepower. The mileage? About the same. This occurs even though the newer car has nearly 2x the engine size and similarly, the new car is much heavier and has a greater frontal area and very likely worse air drag coefficient due to being a SUV.

There is so much going on related to the mileage that you cannot say that car with a bigger engine has always worse mileage. For example, consider these:

• Weight of different cars is different, and heavier cars have worse mileage
• Frontal area and air drag coefficient vary too, with bigger cars having worse air drag
• The cycle of the engine (Otto or simulated Atkinson) affects mileage with simulated Atkinson being better
• If the engine is a diesel engine, it can be very big and still work with great efficiency because diesels are very efficient at part load
• Some cars are hybrids, meaning a battery can provide additional power. For example, gas engines work better if half of the time you have 2x the power (with half going to recharge the battery) and half of the time you're cruising off the battery. Note that no external recharging of the battery (which I would call cheating) is necessary.
• A car may have low aspect ratio tires with big wheels to look cool, but it reduces fuel efficiency due to increased rolling resistance
• Drivetrain losses in AWD and RWD cars can be bigger than in FWD cars, but however, today many AWD cars are mainly FWD with the AWD function provided with electric motors attached to the real wheels
• Transmission may not allow the car to operate at the maximum efficiency point unless it's a mechanical CVT or an electrical CVT (eCVT).
• There are also quite many engine techniques to increase mileage, such as variable valve timing or slightly offsetting the crankshaft. Listing all of them here would make the answer too long.

Typically, however, for gasoline engines bigger engine means worse mileage. That is a good rule of thumb, but here's so much going on related to mileage that you cannot say it is always true.