Automobile fuel economy has become a lot more important over the last few decades. We’ve seen the price of oil based fuels continue to rise, while also realizing that the massive amount of carbon we’re putting back into the atmosphere is harmful. All vehicle manufacturers realize that the consumer wants a more fuel efficient vehicle and they are researching and implementing new fuel saving measures to meet that demand. Years ago it was not uncommon to drive a car that got 10-14 MPG. In today’s automotive market, it’s hard to find a vehicle that is so inefficient with fuel.
Diesel powered vehicles are becoming easier to buy.
Diesel engines are becoming more popular amongst automakers lately. For a long time it was almost impossible to find a diesel powered passenger car in the United States. In the late 70’s Volkswagen started selling their diesel rabbit cars and pickup trucks. These vehicles proved to be popular for their low cost, simplicity, and 50+ MPG. Since then, VW has expanded their diesel offerings in the US and lately has been met with some competition from fellow German automakers BMW, AUDI, and Mercedes-Benz. In the last 5 years or so, a few American made models have been offered with diesel engines as well but none of them have been popular.
Why is the diesel engine gaining popularity?
The diesel cycle engine is more efficient than a typical gasoline engine at converting fuel into mechanical energy. On average, a diesel engine is around 20% more fuel efficient for the same displacement. Diesel engines also tend to last about twice as long as their gasoline counterparts. The longevity of a diesel is in large part due to the fact that it runs at lower RPMs than an equivalent gasoline engine does. Gasoline engines are designed to run between 6000 and 10000 RPMs while a typical diesel engine might only run up to 3000. If you consider that an average diesel engines moving parts are moving at about half the speed of a gas engine, it’s easy to envision how they can last twice as long. Another piece of the puzzle as to why diesels are more efficient is that diesel fuel has more energy per gallon than gasoline does. If you have one gallon of diesel fuel and one gallon of gasoline, you can make more heat or power with the gallon of diesel than with the gallon of gasoline. This is true no matter what grade of diesel, or what octane rating the gasoline is. Diesel fuel just has more energy in it across the board.
The Gasoline powered car
Gasoline has been the fuel of choice for our cars since we’ve had cars. We basically went from horse drawn wagons to gasoline powered vehicles and have stayed there ever since. There were a few early steam powered cars, diesel powered cars, a rumored Nicola Tesla built electric car, and a handful of other oddities. Gasoline came out on top and once we had the entire infrastructure in place to distribute and pump gasoline, it just became “the way things are”.
Gasoline engine evolution
The gasoline engine is the most widely used internal combustion engine in history. Diesel engines are popular for some specialty applications, natural gas, propane, kerosene, and some other fuels are also used occasionally. Gasoline stands atop the competition as the hands down favorite. The gasoline engine has gone through many revisions and improvements over the last 100+ years. It went from a single cylinder air cooled engine to having many cylinders cast into the same block. Current automotive engines have over-head cams, over-head valves, serpentine belts, electronic fuel injection, and many other fuel saving and power adding features.
Here comes the electric car
Electric vehicles are making a huge impact now. Many electric and electric-hybrid vehicles have become almost common to see on Americas roadways. It wasn’t very long ago that there were no electric drivetrains on the road at all. Now you can hardly take a drive without seeing a Prius, Volt, or some other hybrid sharing the road with you. The current darling of the electric car group is the Tesla sedan. The new Tesla models are fine luxury vehicles with all of the high tech comforts you would expect of a normally powered luxury sedan. Almost every single automaker has a hybrid or 100% electric drive train that they offer. Let us also point out that 100% electric cars are not pollution free. They get their power from the existing power grid. East of the Mississippi 60% of our electricity is generated by coal burning power plants. One could make the argument that cars in this part of the country are essentially coal powered cars.
Trucks are not getting much better fuel economy
Not much progress has been made on the fuel efficiency of trucks. Just this year, there are a couple manufacturers that are now offering smaller diesel engines, but they are far behind the progress passenger cars have been making. Getting 25MPG in a pickup truck is something to brag about in advertisements. Getting 25MPG in a car is embarrassing.
Truck MPG Problems
There are two main contributors to the poor fuel efficiency of trucks. The factors are vehicle weight and Aerodynamic drag. Because trucks are used to haul heavy loads, they must be built with steel that is thick enough to support the weight of what will be hauled. This steel, and the associated suspension components, are much heavier that their passenger car equivalents. The added weight that a truck has means that it takes more energy to get it moving from a stop. The lighter a vehicle is the less fuel it uses to get up to speed.
As far as aerodynamics go, trucks are doing everything wrong. Trucks have a big open bed that essentially “cups” the air as you drive. Trucks have a high ride height that increases their frontal area resulting in more aerodynamic drag in the front. Trucks have a wide stance for stability and to lessen the chance of a rollover, but that wide stance also increases the frontal area, again causing more drag. Those are just a few of the big aero issues with trucks. One of the smaller ones that seems easy to fix is the use of the huge side mirrors. These big mirrors are essentially the same as bolting a big paddle on the side of your vehicle to increase your wind resistance and reduce your fuel efficiency. Trucks tend to have wide tires which also affects aero drag. Some wheel and tire packages are available to combat this, but they are seldom used.
Intake air matters
Some advances have been made in the handling of intake air for ICE (internal combustion engines) in recent years. The most popular are the pleated reusable air filters, and the “cold air intake” systems. The reusable air filters such as the ones K&N make claim to give you better fuel economy and more horsepower. They are able to make these claims because the filters they sell are less restrictive to airflow and therefore make it easier for the engine to pull air through them.
The Horsepower and MPG gains from a cold air intake system are made for a different reason. Cold air intakes are called so because they are intended to lower the temperature of the air that is going into your engine. Colder air is denser than warmer air. If an engine is given colder denser air it can expand that air more than air that is warmer, and the additional expansion translates to more power. The way a cold air intake reduces an engines air charge temperature is to relocate where the air is coming from. In a typical engine compartment, the air intake is at the top and under the hood. In this location the intake is drawing in air that has been warmed by the heat of the engine and raised to the top of the enclosed engine compartment. A well designed cold air intake will draw air from a lot lower to the ground and closer to the outside of the vehicle. Since hot air rises, we can easily deduce that the air lower to the ground is cooler. Also, the farther from the engine and closer to the exterior of the car you get, the cooler the air will be as well. Consider that a normal gasoline engine compartment can get over 200 degrees F, and it’s not hard to see how drawing outside air into your engine could drop the temp of the incoming engine air by 100 degrees or more.
Changes in Fuel
Internal combustion engine fuels have remained relatively unchanged in the last century. Gasoline mixtures have only seen two big changes. The first change was the removal of lead from gasoline. That phase out was not completed until 1996. The second change was the addition of ethanol. That change started in the mid 2000’s. Ethanol does not have as much energy as gasoline. Running an engine on pure ethanol reduces your mileage by 34%. A typical ethanol blend in the US is gasoline with 10% ethanol added. This blend will result in 3.4% worse gas mileage(less MPG) than if you were burning gasoline without ethanol added.
The Science of fuel efficiency
The miles per gallon you can drive your vehicle all come down to simple science. There are only a few variables that you can change to get better mileage. You can run your engine more efficiently. You can burn fuel with more energy in it. You can reduce your aerodynamic drag. You can reduce your weight. You can reduce your rolling resistance. Factors outside of those listed are generally not in your control. Those factors would be things like temperature, humidity, elevation, road conditions, and wind.