With gas prices on the rise, and average incomes dropping lower and lower, I'm sure pretty much everyone is concerned with saving fuel. Even if you're not doing it for the environment, you're doing it for your pocket. Most people say fuel economy and horsepower can't coexist. This is not true. The reason they haven't cooperated with eachoter has been due to the auto manufacturers approach to "efficiency."
The biggest problem is this word efficiency. The easiest way I can describe it, is there are two separate views of efficiency when it comes to motor vehicles. The first (and most common) is vehicle efficiency. This is simple to figure out, just find your gas mileage. The other way to look at it is engine efficiency. This aspect is most often overlooked when shopping for an "efficient" car. Most people would say that your average Honda Civic getting 30mpg is more efficient than a big rig getting 10. This, however, is a falacy. In fact, that big bad diesel is much more efficient. The truck is getting a much larger "boom" per unit of fuel than the little gas engine is.
How, you may ask, can you get that little 4-cyllinder to get as big of a boom out of that fuel as the diesel? Well, technically it's impossible. Since diesel fuel stores more chemical energy than gasoline, the perfect diesel engine will always be more efficient (per unit volume of fuel) than the perfect gasoline engine. But let's first skip the technicalities for the moment and look at the fundamental differences between the diesel engine and gasoline engine.
There are a few key differences between the diesel and the gas engine. First is fuel delivery. Most diesels incorporate some type of direct chamber injection, where the fuel is squirted straight into the combustion chamber just when the time is right for the fuel to burn. This gives the engine a definate advantage, since the timing of combustion can be precisely controlled.
On a gasoline engine, you have a few options. You can have carbuerator, throttle body injection, direct port injection, or chamber injection. Chamber injection is just like the diesel (from above) and is generally the most efficient but is also the least common (and most expensive). I think some VW's and maybe a Honda or two incorporate direct injection.
In a carbuerator, the air that is injested by the engine is sucked past what's called a "jet" that, by means of a difference in air pressure, draws fuel droplets into the air stream. This air fuel mixure (will be reffered to as A/F mixture) is drawn into the intake manifold where it is distributed to each respective cyllinder. The intake stroke of each cyllinder is what causes the difference in air pressure.
Port injection (DPI) utilizes a pump that pressurizes the fuel through lines to "injectors" that, when stimulated by an electrical pulse, allow some of the fuel to be released. The speed and duration of the pulses determines how much fuel is squirted into the port.
Throttle body injection (TBI) is a hybrid between the carb and the port injection. Instead of having an injector for each cyllinder, the injector replaces the jet in the carbuerator and the fuel is electronically metered on top of the engine, where it is drawn through the intake manifold and fed to the cyllinders.
Historically the carb has been known as the least efficient, followed by throttle body injection and then port injection. The key difference between these systems that causes differences in efficiency is the metering accuracy. Since TBI and DPI are electronically metered, the onboard computer can accurately controll the amount of fuel that enters the engine at any given moment. A carbuerator is metered by the physical aspects of the carb itself, which cannot automatically adapt to differences in RPM, temperature (other than a "choke") or quality of fuel, in the way that the comupter controlled systems can.
Getting back to the efficiency aspect of these engines... The other main difference between a diesel engine and a gasoline engine is the compression ratio, CR. The compression ratio is the ratio of the volume of the cyllinder when the piston is at bottom dead center (BDC) to the volume of the cyllinder when the piston is at top dead center (TDC). [If you don't know what I'm talking about when referring to cyllinder, piston, TDC, BDC, etc that will have to be another topic because I could write an entire book describing it lol.] Typical CR's on a gasoline engine range from 9:1 to 11:1. Race engines push that further into the 12:1 or even 13:1. Diesels, on the other hand, are running CR's in the 25:1 range.
There are two aspects that increase efficiency with a higher CR. The first is the compression. When the air and fuel are compressed they get hot. This is due to Boyle's law that says that when you compress a gas its temperature goes up. With the increased heat, the more quickly and completely they will combust. The second aspect is the Expansion Ratio, ER. This is the exact opposite of the compression ratio. It deals with how far the now burned [read HOT] gasses can expand. There are limits, but basically the more time the hot gasses can push on the piston, the more the heat will be converted to physical energy. The expanding gasses is what does all of the work inside an engine.
Since nothing is instant, this burning has to take place slightly before the piston reaches TDC. This is referred to as ignition advance (or spark advance). It takes a little time for the actual combustion to start, it takes a little more time for the heat to be transfered from the burning fuel to the nitrogen in the air (the nitrogen gas is what is expanding) and it takes even more time for that pressure to "push" against the piston. This is why the spark plug (ignites the fuel) is fired before TDC.
Due to the extreme pressures and heat created by compressing the oxygen and fuel together, sometimes the fuel simply doesn't want to squeeze anymore and prematurely ignites, starting the combustion process too early in the compression stroke. This is called preignition, or "ping." If left unchanged, pinging will knock holes in pistons, burn valves, "throw rods" and do all sorts of other nasty things to your engine. It also reduces power. Since it is firing too far before TDC, the piston that is trying to compress the air is simultaniously being pushed back down by the expanding air. The engine is basically fighting itself.
The most common way of eliminating ping is running a higher octane premium fuel. This fuel is a bit more stable, so it resists preignition. Once it finally does fire, it burns just as quickly as "regular" gasoline. This allows you to keep the same timing on a high CR engine as you would on a lower CR engine. While it is a simple fix, it has one major flaw. Premium fuel is more expensive than regular fuel. On to the next fix...
With a high CR, some of the fuel is turned into large droplets of liquid (due to Boyle's Law). Due to the added heat resulting from the high CR, the fuel that is still vaporised burns much more quickly than "normal." Due to ignition advance, this quick burn manifests as preignition. The most common way to deter ping (other than high octane fuel) is to retard ignition (spark) advance. This makes the vaporised fuel burn when it needs to in the compression stroke, transfering the power to the engine. However, since there is still liquid fuel in the combustion chamber, the liquid is either slowly evaporated and burned later in the power stroke, pushed past the rings, or simply pushed out the exhaust. Wasted. This is why you lose power and efficiency on an engine when retarding the ignition (less fuel is turned to power). It is also why engines with a lot of cam overlap tend to idle poorly. That unused fuel burns during the exhaust stroke and that added pressure transfers through the combustion chamber during overlap into the intake manifold. To eliminate these problems, we need to eliminate the liquid fuel droplets and ensure all of the fuel is in a gaseous state upon ignition.
There have been discoveries pushing engine efficiency to the next level. Their key goal is to keep the fuel vaporized. Here are a few links to websites that have been pushing this technology foreward.
[url="http://www.somender-singh.com"]www.somender-singh.com[/url]
[url="http://www.mpgresearch.com"]www.mpgresearch.com[/url]
[url="http://www.fueleconomytips.com"]www.fueleconomytips.com[/url]
The first link (Somender Sing's website) is one of the most interesting, yet very simple way to increase combustion efficiency within an engine. This simple modification increases the rate at which the fuel burns inside the engine, thus greatly increasing thermal and mechancal efficiency.
Think of it this way... You've got a crossbow and a compound bow. Both have the same draw weight. The crossbow will be much harder to pull than the compound bow, but the compound bow will out-shoot the crossbow every time. That is because all of the pressure is exerted by the bow is in a very small area (at the very end of the pull). This concept is the same for your engine. If you can get all of the pressure from combustion to push on the piston as soon as it goes over TDC you will get much more torque, power, and efficiency.
Gasoline vapor out in the air (very little turbulence) burns very slowly, in the low meter per second range. Obviously this wouldn't work well within an engine, because the piston would already be through a complete cycle before the A/F mixture is burned. The grooves achieve a quicker burn by inducing turbulence into the A/F mixture. As the piston approaches TDC, a strong jet of air is forced out of the groove and into the [large open] combustion chamber. What liquid fuel that has formed due to the process of compression is broken up into much smaller droplets and is much more evenly mixed throughout the open combustion chamber. Due to this thorough mix, the fuel can much more readily be vaporized and burned much more quickly in the compression/power strokes. Also, since these tiny droplets have so much more surface area, there is more fuel in contact with the hot air charge and the hot engine components. The liquid fuel absorbs the heat from the air charge and piston/head to evaporate even more completely and stays in a vapor state for much longer during compression. This raises the thermal efficiency, since the heat that would normally be emitted through the radiator and exhaust is doing work inside the engine, vaporizing the fuel droplets. This also equates to a very quick burn. Remember the compound bow? We're getting closer...
If it hasen't become clear by now, the next "breakthrough" has been inferred: Liquid fuel doesn't burn. Only the vapors (fuel that is evaporated and greatly mixed with the oxygen in the air) will burn and create power. The Singh grooves help mix the fuel into the hot air charge which helps the fuel to evaporate.
On MPGResearch, other things have been developed as well. PowreLynz(tm) introduce another way to increase the evaporation of that fuel. They're basically 20-pitch threads screwed into the intake tract of the head. This both increases flow and evaporates the fuel. The flow increases because it eliminates "laminar flow" that slows air delivery. Also, since there are no layers of slow-moving air, the fuel droplets on the outside of the mixture slam against the threads and are broken into smaller droplets that can be more readily evaporated. Even the fuel that doesn't slam and break up is "wicked" into the threads and evaporated from the heat within the metal of the head itself.
These two mods greatly increase thermal and mechanical efficiency and therefore have huge horsepower potential. Don't forget the fuel economy side of things, too, which was originally why they were developed. Since you're getting the most power out of the amount of fuel delivered, you can greatly lower the total fuel used (how hard you push on the pedal) to get the same speed.
This post got a whole lot longer than I expected when I started typing, but for a recap, here are the websites again.
[url="http://www.somender-singh.com"]www.somender-singh.com[/url]
[url="http://www.mpgresearch.com"]www.mpgresearch.com[/url] <-- Was recently hacked and is temporarily down
[url="http://www.fueleconomytips.com"]www.fueleconomytips.com[/url] <-- Was also hacked and is down
Apparantely the last two were a strong enough threat to someone that they were attacked and cracked. They should be back up shortly.
If there is anything I should change, let me know. Maybe I should break it up somehow into a few different posts. Let me know!
Thanks, Bindusar (from the OverClock.Net forum), for your input regarding compression, Boyle's law, and fuel condensation during compression.
