Showing posts from October, 2011

Engine Efficiency #3

The third and perhaps the most important efficiency related to engine performance and affecting overall engine output is volumetric efficiency. This is essentially a measure of how easily the engine breathes air in and out. The more air that can be moved into the engine, the more oxygen there will be to mix with more fuel. This creates a more powerful combustion event whenever more power is needed. The power is in the fuel. If you can burn more fuel you can create more power. The actual rating of volumetric efficiency is the measurement of air that actually makes it into the cylinder while the intake valve is open, and the piston is moving down, expressed as a percentage of the theoretical potential volume of the cylinder. To understand volumetric efficiency it must first be understood that the piston moving down on the intake stroke, with the intake valve open, only creates a negative pressure within the cylinder; it does not suck the air in as you might believe. Once negative pressur

Engine Efficiency #2

The next efficiency that has an effect on the way an engine performs is thermal efficiency. As you can imagine this has something to do with heat, that is to say heat not temperature. In case you are not quite clear on this subject, heat is a reference to one of the basic forms of energy, and we typically measure this in Joules or British Thermal Units (BTU). Temperature is only a measure of the intensity of heat energy and we measure this in degrees Celsius or Fahrenheit depending on your unit persuasion. One joule of heat is equal to the amount of heat given off by the human body at rest over a period of about 15 seconds. The temperature of the human body is 37° C, but if you were to take that 1 Joule and spread it out over the size of a medium sized living room, the intensity of the one Joule of heat would drop significantly so that the temperature of that room would be something very cold. Gasoline is burned in the engine in a combustion process that converts the chemical energy of

Engine Efficiency #1

Many things go into the design of an engine. Engineers must consider what kind of vehicle the engine will be used in, and this means everything from, how heavy the car will be, how many people it will haul, how much stuff it will haul, what will the overall volume of the vehicle be, and so on. Usually they will try and apply one engine to as many different vehicle platforms as possible in order to make the money spent on development go farther. This is why the V6 engine found in the Lexus ES350 bears a striking resemblance to the engine found in the Toyota Sienna, or the engine in the Pontiac Solstice seems just like the engine in the Chevy Cobalt. When engineers adapt the same engine to multiple platforms they often make a few changes to the power plant that make it more suited to the platform. These changes affect the output of the engine because they usually change one of three efficiencies that ultimately determine how that engine will perform. These efficiencies are volumetric eff

Good Explosions

Tiny explosions occur inside the engine and that’s what makes it go. Squirt some gas into the engine at just the right time, light it off, and you’re ready to go. This is true enough but the best way to describe the combustion that takes place inside an engine is as a process of energy conversion. Convert energy from a form that is cheap, portable and easy to store, into a form that will move us down the road. That is what the combustion process is all about. A few simple things are needed to make this energy conversion take place smoothly: spark, fuel, and compression. That’s it; nothing else is required, except for maybe the proper timing of these three things. All of these must happen at the right time, or at least close to the right time and the engine will run. In order to have compression, the intake and exhaust valves must be closed, and the piston must be moving in an upward direction within the cylinder. This is easy enough, what about spark and fuel? Fuel The fuel is where th