Engine requirements for 100 mpg

[Ernie Rogers]

Hello, folks,

I thought it might be useful for some to define in a mathematical fashion what the requirement for an IC engine is, in terms of efficiency, to obtain 100 mpg with the engine only.

A calculation like this is built on assumptions. I assume a car running at sea level at a constant speed-- then I will seek to find what IC engine efficiency is required to keep above 100 mpg. Here are the other assumptions I will use:

Frontal area: 24 sq.ft. = 2.23 m^2
Drag coeff: 0.20
Air density: 1.22 kg/m^3
Head wind: 0
Car weight: 3100 pounds, with fuel and full load of passengers
Rolling resistance: Crr = 0.006
Efficiency from shaft to wheels: 0.85
BTU /gallon: 116,090 BTU /gal

With this information, and knowing the rate of fuel consumption is 100 miles per gallon, I can calculate the needed engine efficiency. Here are the results, for a range of speeds:

SPEED....EFFICIENCY
...50......... 0.34
...55......... 0.385
...60......... 0.435
...65......... 0.485
...70......... 0.54
...75......... 0.605

The calculations were made using a spreadsheet calculator I have -- I will send it to anybody that requests it.

Ernie Rogers

[Matt Timion]

This is great information Ernie. it's amazing to see how quickly the efficiency requirements increase as the speed increases.

Oh yeah, and welcome back

[omgwtfbyobbq]

And just to put those engine efficiency requirements into perspective, peak BTE (brake thermal efficiency) for a 1.9L TDI is brushing up against 41%, a Prius roughly 40%, with the average car being at about 35-40%. Gearing suitable for cruising at some predetermined speed with minimized BSFC would allow most vehicles on the road to achieve Prius/TDI like efficiency.

[Sludgy]

A large part of the reason that real world mpg numbers are so low is that the engine rarely operated at its best efficiency point. At idle, it's 0% efficient, and in stop and go traffic where most of us drive, we never see anything close to 100 kph.

Although a good engine can do wonders for fuel economy at steady speeds, it can't help wasting energy at idle and stop/go.

[northboundtrain]

According to your numbers, 50% efficiency would be required to achieve 100 mpg at realistic highway speeds (This in a vehicle with exceptional aerodynamics similar to the Mercedes Bionic concept car). Current engines can't approach that kind of efficiency due to the cooling requirements of the engines' materials. Steel and aluminum expand too much to maintain the required tolerances.

I remember reading something about the possibility of ceramic engine components that could handle much higher temperatures without significant dimensional changes. Apparently a ceramic engine block with ceramic pistons wouldn't even require piston rings because the piston could be so closely sized to the cylinder. Such an engine could operate in the 400-500 deg F range and would need no cooling system.

[s2man]

I've read that ICEs are most efficient at their point of maximum torque. Is this the same point as peak BTE?

[retrorocket]

In the past I have been around some Nascar teams and they said that they could get around 4.9 MPG with an average lap speed of the high 180's.

What would that translate to with all the factors for mpg at 60 mph?

retrorocket

[basjoos]

I have no problem maintaining 89mpg at 65mph when the temps are in the upper 80's F. I wonder how much better my mileage would be if I didn't have an engine with 240,000 miles on it with low compression (or if I had the VX ICE or a Kubota diesel installed).

[omgwtfbyobbq]

-Max torque used to be around peak BTE for SOHC engines, but with the relatively flat torque curves of modern DOHC engines this isn't the case. A good rule of thumb is that peak BTE will be at ~2-3k rpm and near peak load.

-High speed race cars are generally at near peak efficiency because the force due to aerodynamic drag is maybe 40 times greater than when cruising at 60mph, if they were to cruise at 60mph, engine efficiency would drop, but vehicle efficiency would increase, and they'd probably be in the 20-40mpg range imo.

-High mileage really won't hurt efficiency much ,unless the engine is literally bleeding oil. If we increase our vehicle efficiency, we'll decrease engine efficiency, and can't maximize engine efficiency and vehicle efficiency w/o changing the gearing.

[Bill in Houston]

Northboundtrain,
You and I must have read the same article in something like Popular Science 25 years ago. Still no sign of ceramic engines. I love the idea, though.

Basjoos,
It seems to me that a car with aerodynamics like yours, a 40 hp peak ICE, and 40 hp of electric motor, could easily see 100 mpg and be very pleasant to drive in all situations.