Article Title
Article Title

Combusting Engines

by Josh Zeisel

One of the most popular topics in our society is cars. Men love to fix 'em, women love to check out guys in sweet rides, and teenagers always seem to be dreaming about what car they want to buy after they get their license. Look around and you'll undoubtedly see a car in your general vicinity. More likely than not you've driven a car, been in a car, or at least know someone who owns a car. We all know what makes cars tick. We like how they sound and we are disappointed when they don't provide the power we need to accelerate from 0 to 60 in 2.87 seconds. The internal combustion engine is to a car as a rocket off the Green Monster is to a single. It's inefficient and there are better options. There are the Yankee Stadium of options compared to internal combustion engines.

Before I can go off on why there are better options I have to explain what goes on inside an engine so we are all on the same page. Any engine you'll ever come across (be it car, jet, or steam) has the same basic cycle. A fluid is compressed, energy is added, and mechanical work is done to get the objet in motion. Steam engines are external combustion engines, while car and jet engines are internal combustion engines. For simplicity, we'll only focus on the internal combustion engine, or "ICE". Other engines will get a nod, but since this is an article about cars and engineering, we'll keep it practical. An ICE compresses air in a chamber, adds gasoline -- whether it be in a cylinder like that of a car or in the combustor like that of a jet engine -- ignites the air-fuel mixture, and finally makes something move. In a car, the cylinder head is forced up, which turns the crank shaft that is transmitted to the wheels through the transmission. In a jet, the turbine is turned by the ignited gas that flows which turns a crankshaft that powers the compressor so the cycle can start over again.

The efficiency of an engine is based on a few things: The optimal air-fuel mixture, how hot the gasoline burns, and the transmission of power are the main factors. In a car, the mixture is computer controlled. Most of the time you see the check engine light come on in your car (at least the more modern cars on the road), it's because you probably need to change the air filter or the oxygen sensor. The air filter is just that: it takes all the crap out of the air (especially all that sand on the Long Island Expressway) to make the engine run smoothly. The oxygen sensor measures how much oxygen is going into the engine; the stuff that makes things burn. The less oxygen in the mixture the less efficiency you get out of the ignition cycle.

The computer controls how much air is let into the cylinders at any given time. This changes due to humidity, temperature, and altitude. If you've ever been flying in an aircraft with me (have you not?), you may notice me playing with the third red nob (or lever, depending upon the plane). This directly controls the mixture. I can see how much fuel is being put into the engine, measured in gallons per hour. When the plane goes up, you need less fuel because there is less oxygen, so you pull the lever down. When you land, you better push that lever back up or else the engine will stall from the lack of juice.

Jet engines do exactly the same thing, just in a more intelligent way. Jets first compress the air that travels into the igniter rather than compressing the mixture together at the same time, like in a car. Compressing the air before fuel is added allows more oxygen to exist in the same space as if it wasn't compressed. It allows jet airplanes to fly higher than the dinky Cessnas and Pipers of the general aviation world. The compressor can effectively compress the high altitude air enough that it can attain proper mixtures to fly where there is less oxygen. And, yes, the amount of fuel pumped into the igniter is computer controlled. A proper mixture allows the ignited gasses to burn hotter, which increases the output power of the engine.

The highest efficiency an engine can attain is based on something called a Carnot Cycle. The hot gas inside a housing pushes on a piston. As the gas expands, it cools off, allowing for that cool gas to get compressed by the piston coming back around to the original position. Compression increases the heat of the gas starting the cycle over again. In a Carnot cycle, it is assumed that no heat is gained by, or lost to, the outside environment. This would be great if we could find a perfect insulating material...but we can't. We also can't reduce the friction of the spinning piston to zero, which turns mechanical work into heat. And according to the Second Law of Thermodynamics, you can't turn 100% of that heat back into mechanical work.

This explains why aircraft engine manufacturers are always trying to attain hotter ignition temperatures. The hotter the gas, the more expansion can take place. When this expanding air flows past the turbine blades which are tiny airfoils (the shape of an airplane wing that allows it to create lift), more lift can be created, which means more turning power on the engine shaft. All that power is transferred through the shaft back to the compressor to compress more air.

So what's the efficiency of a car engine against a jet? Would you guess 90%? 80%, maybe even 50% considering the disappointed look in my face that dropped you from 80%? That seems reasonable. Well it's wrong. Try about 20% for your fuel efficient Honda and about 30% for that Boeing 747 that just flew into your local international airport. But why?

Well for one, the gasses that come out of the muffler and the back of a jet engine are still hot. So they never were able to fully expand and cool back to their original temperature. Secondly, there is friction. There is friction between every moving part inside the engine, which turns mechanical energy into heat which, as we learned before, is not fully reversible back to mechanical energy. For reference, a rocket engine attains 70% efficiency, which is great, but it would be really hard to control that speed within a rocket.

So why not use a jet engine inside a car instead of that 4-cylinder Honda Fit engine that you think is so efficient? If you can get 30 miles to the gallon out of an engine that is only 20% efficient that means with a jet you'd be able to get 45 miles to gallon out of 30% efficiency. At an average gas price of $3.815 per gallon, that's a savings of $57.225. But to create the same amount of horsepower, a jet doesn't need to be as large as a car engine. This allows the car to be more streamlined which decreases drag. The engine will also weigh less which will increase the power to weight ratio and get you better accelerations off the line.

There are two main reasons why we don't use jet engines inside of our cars. People want cars that look like cars. If we streamlined them, they'd look like airplanes. The other reason is that jet engines are incredibly complicated. Trying to explain them so the layman would understand is hard enough (If I did a bad job, I apologize). Think about your local mechanic: do you want that guy working on something that complicated? You probably don't even want him working on your car as is.

Forgetting about efficiency for now, let's focus on getting more power out of that Honda Fit engine. This can be done in a couple easy ways. The cylinder size can increase, which would allow more fuel to be pumped into the cylinder. Another way is to turn that 4-cylinder engine into a 6- or an 8-cylinder engine. The last way is add a turbocharger. The turbocharger compresses the air before it goes into the cylinder to mix with the fuel, á la a jet engine. In the past, turbochargers used to turn on whenever you stepped on the gas pedal. Now, the computer knows when you really want more power and turns the turbo on when needed. Turbos use more fuel because the engine recognizes that there is more oxygen to take advantage of and pumps more fuel into the cylinder. Computer controls reduce the time the turbo is on and decreases the total amount of fuel used over non-computer controlled turbos.

Currently, hybrid cars use a small ICE to assist the electric motor. The ICE recharges the battery or adds that little extra power for a speedier acceleration. As we've learned before, these regular ICEs, even small ones, aren't very efficient, but turbos allow the engine to be closer to that of a jet. This is the idea behind a new type of rotary engine being developed at the University of Michigan.

The engine is made up of a stationary chamber and 15 spinning rotors, or blades. The chamber is split up into four sections. Two sections allow gas out towards the outside of the chamber, at the center of the chamber, and the other two are where the compression of the mixture and the ignition of the mixture take place. The blades spin inside the chamber pushing the mixture towards the outside using centrifugal force. As the new mixture is coming in at the center, the old spent fuel is leaving at the outside of the chamber. The rotors spin and push the new fuel to the compression section. The fuel slams against the outside wall of this section, which creates a shockwave that compresses the mixture. The mixture is then ignited and pushes the rotors and makes them spin. The rotors spin and push the spent fuel to the exit all while being replaced in the same step by new air and fuel. The engine is very small, only about the size of a frying pan. It potentially increases fuel efficiency by three times and is perfect for hybrid cars that don't need an engine as their primary power source. Unlike a traditional gas engine, there are less moving parts making it more reliable and potentially easier to repair.

Oil is a great commodity. People make billions of dollars a year processing and selling different types of oil-based products. It is also a limited commodity whose use works against our environment, leading us to need new sources of power. Unfortunately, to attain the efficiency of a gasoline engine out of new fuels this will take time, but in the meantime we have viable options to decrease our fuel consumption. Options that are only possible if we make sacrifices and spend a little extra on the front end for our rebates on the backend of our investments.

(Image courtesy of Accretion Disc)

 

Josh Zeisel is a professional mechanical engineer and graduate of Boston University. His favorite meal is a chicken parm sub and an orange soda. On clear sunny days you might look up and find him flying something. Strike up a conversation with Josh at josh.zeisel[at]theinclusive.net