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Engine fuel consumption factors and considerations: An engineering analysis for lay persons

Clarke Echols

A number of bus nuts have been asking questions along the line of, “If I have a ??? engine, what kind of fuel efficiency can I expect?” This is a complicated question that is not easily answered without a lot of information about various factors in the vehicle being considered. I will address some of these factors and explain how they relate to fuel efficiency. The information I present is based on my experience in interpreting the fuel consumption curves for Detroit Diesel products. The principles also apply to other manufacturers’ engines, but the specifics vary both with product and specific model designs.


Fuel-consumption-versus-engine-speed curves for Detroit Diesel engines equipped with electronic controls are readily available, but the same information for 6V71 and 8V71 models is limited. Specifications that are available for 71-series models indicate a fuel consumption of about 0.39 pounds of fuel per horsepower hour with a size 65 injector at 2100 RPM. In comparison, the fuel consumption for a 320-HP Series 50 engine at the same speed is 0.34 pounds per hp-hr; i.e., an 8V71 burns about 15% more fuel than a DDEC-equipped Series 50 under similar conditions.

However, keep in mind that the 6- or 8V71 also is usually air-aspirated (no turbo-charger) and fuel-consumption numbers will vary radically from the stated values at high altitudes, such as on Colorado mountain passes where air density is dramatically lower than at sea level, although the governor may play a role in limiting over-fueling at any given throttle setting. By packing more air into the cylinder, a turbo-charger substantially reduces the effect of altitude on engine performance, and the DDEC computer also compensates by injecting less fuel when pressure is less due to altitude. While the presence or absence of turbo-charging or electronic controls can dramatically affect fuel consumption, especially at high altitudes, the general principles explained in this analysis still apply.

Engine Fuel Consumption:

Diesel engines consume a roughly constant amount of intake air per revolution regardless of RPM and power demanded by the load. They do not have a throttle butterfly in the air stream like you find on the intake of a gasoline engine. The amount of power delivered on any given cycle is directly related to the amount of fuel injected into the hot air at the top of the compression cycle.

When the fuel is sprayed into the chamber by the atomizing injector at the top of the compression cycle, it immediately catches fire and begins burning. The engine is most efficient when the fuel is completely burned just as the piston reaches the bottom of the power stroke. If the engine is turning slower than the optimum speed, the fuel is spent before the piston gets to the bottom forcing it to coast for the rest of the cycle, and if turning faster, the piston gets to the bottom and the exhaust valves open while the fuel is still burning. Either of those two latter conditions causes the engine to burn more fuel because you are not getting the work that is available in the burning fuel during the entire and only the entire duration of the power stroke. Anything more or less than this is wasted energy or wasted opportunity to get energy because to get the power you need, you increase the amount of fuel being injected by pushing down further on the throttle pedal.

The amount of fuel consumed by the engine is measured in pounds of fuel per brake-horsepower-hour. In other words, fuel consumption is directly related to power out of the engine, regardless of vehicle speed. Fuel used per hour is tied to horsepower required to drive the load. The fuel consumption curve is dish-shaped, with the minimum (most efficient operation) fuel usage in the range from about 1500-1600 RPM. When you operate outside of that speed range, fuel consumption increases whether you reduce or increase the RPMs. At 2100 RPM, it is about 10% higher than at 1600, and at 1200 RPM, it is about 10-13% higher at the same delivered horsepower.

Horsepower Required:

The amount of horsepower required to drive a coach consists of:

bulletPower required to run the engine, transmission, air compressor, alternator, radiator fan, etc.
bulletPower required to overcome rolling resistance.
bulletPower required to punch a hole through the air (aerodynamic drag).

As a rule of thumb, the first item consumes about 30-45 horsepower. The second varies with type of road surface (smooth pavement takes less power than chip-seal or rough pavement, and gravel roads take even more). Again, plan on about 40-50 HP for a 35,000 pound coach at 70 mph on typical interstate highway. Between the two, you’ve shot about 75-85 HP. The third factor depends on the frontal cross-sectional area of your bus and the aerodynamic streamlining. It also is dramatically affected by road speed combined with wind speed and direction. For a typical coach traveling at 60 mph, the air drag is somewhat less than rolling resistance-about 35-40 horsepower. Given these base figures, you’re looking at about 110 -120 HP. At 1500-1600 RPM for a Series 50 engine, you use 0.3 pounds of fuel per hour per horsepower, so 0.3 x 120 = 36 pounds. Assuming 7 pounds per gallon, that’s roughly 5 gallons to travel 60 miles, or 12 miles per gallon. Kick the speed up to 70 (at the same engine RPM), and you’ll be burning closer to 7 or 7.5 gallons per hour, or about 11 miles per gallon.

If your engine speed is 1600 RPM at 60 mph, it’ll be close to 1900 at 70, and your consumption per hour will be closer to 8 gallons per hour, or about 10 miles per gallon. Thus, between engine efficiency loss at non-optimum RPM and additional air drag, going from 60 mph to 70 can drop your fuel economy by close to 20%! That’s a LOT, and that is why it is important to select the rear-end ratio and tire size to run the engine in the 1500-1600 RPM range at your typical cruising speed when possible.

How Transmission Ratios Affect Efficiency:

The Allison B-500 transmission (6-speed World Series) gear ratios are arranged such that if you select tire size and drive-axle ring-and-pinion ratio to cruise at about 70 mph in 6th gear at an engine speed of 1600 RPM, you will be running close to 60 in 5th gear at the same engine speed, and close to 45 in 4th (direct). 3rd gear puts you in the mid-30s, and you have a very nice combination for most driving. In addition, at 2100 RPM in 4th gear, the top speed is close to 58 mph, giving you an excellent engine speed range for climbing mountains. Greyhound Bus Lines locks out 6th gear in their MC-12s (similar to MC-9s), giving their drivers a top speed just over 70 in 5th gear. In the process, they give up about one mile per gallon due to high engine speed compared with what they’d be running if they enabled sixth gear and used a vehicle speed limit in the computer software for sixth gear in the Allison. These effects, combined with high idle time on their engines, reduces their fuel efficiency to closer to 8-9 miles per gallon or less if they make a lot of time-consuming stops.

The 4-speed Allison HT-740/748 series, with a drive-axle ratio of 3.3-3.5 yields a top speed in the 69-74 mph range with tires sized in the 480-515 revolutions per mile range with the engine running 2100 RPM at maximum governed speed. Using larger tires (11R24.5) at 483 revs/mile and a 3.55 drive-axle ratio, an optimized engine speed of 1650 RPM produces a tire speed of 465 RPM, for a ground speed of 57-58 mph. Top speed for this configuration at 2100 RPM is just over 73 mph, meaning that if you run the engine that hard, you will likely run less than 10 mpg, whereas the B-500 could yield closer to 11 or possibly 12 mpg in similar conditions. However, in 3rd gear, the speed range is about 40-50 mph with engine speeds between 1600 and 2050 RPM, making it a good hill-climber too. Be aware, though, that if you use this transmission, you won’t likely see 10-13 miles per gallon at 70 mph because you cannot develop enough transmission output-shaft speed to get the engine RPMs down to an efficient range at interstate-highway speeds.

Driving Style Affects Mileage

You’ve seen trucks going down the highway spewing black smoke out of the stacks because the driver has pushed down on the throttle to give the engine more fuel than it can convert into horsepower. When an engine is already delivering its maximum horsepower, giving it more fuel will not provide more power. If you increase the injector size to deliver more fuel, you won’t get more power if the engine cannot deliver the power contained in the fuel being consumed (actually the fuel contains energy, which is power exerted over time). The engine is already burning all the fuel it can, given the amount of air coming into the intake manifold, and giving it more fuel only creates smoke (unburned carbon) which means you are wasting energy.

Thus, if you floorboard the throttle or otherwise try to get the engine to produce beyond its capabilities, it will simply voice its objections by blowing carbon up the stack or out the exhaust pipe at the side of your bus. That wasted energy is also fewer miles per gallon. Modern computer-controlled engines have software that recognizes the engine’s inherent limitations, and therefore limits the amount of fuel provided to the injectors to prevent smoking. That is one of the big reasons why most electronic engines produce higher miles-per-gallon figures than mechanically injected engines.


From this simplified discussion of general fuel-efficiency considerations, it should be rather obvious that you cannot simply ask the question, “If I use engine X and transmission Y in my bus, what kind of fuel consumption can I expect?” Also, even though overdrive transmissions can yield big returns in terms of fuel savings, if you put $10,000 into a transmission to improve mileage by 3 miles per gallon, you’ll have to put over 300,000 miles on your rig to get your money back if fuel is $2.00 per gallon and you get 8 mpg on the non-overdrive transmission (if fuel is cheaper, you’ll have to put on even more miles to recover your investment). If you put the same money into an investment at 7% interest, you can drive over 2,000 miles per month, average, before you reach the point where the interest won’t pay your extra fuel costs without the overdrive gear box. So unless you do a LOT of driving, getting caught up in modest fuel savings is not very worth-while in terms of financial pay-backs.

Observations about Detailitis and the Paralysis of Analysis:

Engineers are trained to watch details. Many engineers have very analytic personalities, and have to have “all the facts” before they can make a decision. One of my engineering professors said, “Engineering is the ART of KNOWING HOW to avoid difficult issues.” After 30 years of engineering experience, I have observed that those are very wise words. My analysis above covers the subject, but avoids the extreme difficulty of examining every detailed factor in the complex equations that govern fuel efficiency and consumption. By using sensible approximations, we were able to cut to the chase and determine feasibility without getting bogged down in details that would have little effect on the actual outcome.

Moral of the story? Don’t sweat the small stuff if it isn’t significant to the big stuff. But sometimes small stuff can cause an accident or major disaster (don’t forget space shuttle Challenger) and in those cases you better sweat the small stuff. It’s all relative...