Compress this!

So, Charles Law as derived from the Ideal Gas Law defines the relationship between the volume and the temperature of a gas. For our purposes, when a gas is compressed, it gets hotter. So it goes to say, if you compress it enough it will get hot enough to burn fuel. This is the whole concept around which all compression ignition engines are based. I'll assume you also understand the basic steps of a four cycle engine: Intake, Compression, Power and Exhaust. In this respect, compression ignition is no different than the spark ignition engine. It's all in how you feed your fire.

Regular gasoline engines use the same terms for each of the four cycles in the internal combustion process. That's about all they have in common though. Starting with the Intake phase, the gas engine pulls in fresh air and fuel vapors in a premixed form into the cylinder. The fuel is metered into the air stream in a predetermined amount to provide a suitable mixture for combustion. In a compression ignition, only fresh air is drawn in.

The Compression phase begins with the piston moving upward and decreasing the volume of the cylinder. In gas engines, the fuel/air mixture will be squeezed so that it occupies a volume anywhere from 1/8 to 1/10 its original volume. This is where the term "Compression Ratio" comes from. As you probably guessed, the compression ignition design gets it's name from this portion of the cycle. The fresh air in the cylinder is compressed to nearly 1/25 the original volume. If you're wondering why either engine needs compression at all, remember that if nothing moves when we light our fuel, there is no work done. No work, no power and we're back to the ox-cart! The compression phase is essentially like stretching a rubber band, we're preparing the system to do something. Gas engines use a much lower compression ratio for several reasons. One of which is to avoid overheating the premix and causing premature ignition. This is referred to as "knocking" and can be heard as a pinging sound outside the engine. It can cause extremely high pressures to be developed in the cylinder and lead to engine failure if not corrected. The much higher volatility of gasoline is another reason for keeping compression down again for same reason. We'll use kerosene in our compression ignition engine for the sake of discussion. Kerosene is much less volatile than gasoline. It doesn't vaporize as readily and thus is more difficult to ignite. Of course because we haven't added any fuel to our cylinder yet anyway, there is no risk of pre-ignition. Now, in someone's immortal words:
"Let's light this candle!"

For the sake of convenience, I'll discuss this next step right when the piston reaches the top of its stroke. In reality, the ignition occurs slightly earlier but let's not complicate things. Upon reaching the top of the compression stroke, the gas engine's electrical system triggers a spark in the cylinder. Whether it comes by breaking the contacts of an ignitor or by jumping across the gap of a sparkplug is irrelevant, it's still a spark. Once lit, the mixture burns until the quantity of mixture capable of supporting combustion is gone. In our compression fired beast, things are slightly diferent. When the piston reaches the top of the stroke, the much higher compression ratio means that the air is being forced to occupy a much smaller space. According to Charles Law, we can put ten pounds of stuff in a five pound container but it is going to get hot in there! The speed at which the compression takes place is such that the heat cannot dissipate and the temperature of the air rises rapidly. The Thermoil manual states that pressures of 425 psi and temperatures of 1000^oF are attained in the cylinder. Keep in mind that paper ignites at 454^oF and metal objects begin to show a discernible color at around 1000^oF, obviously things are getting a little hot in there! At this point fuel is forced into the cylinder by some external means. Once the fuel is injected into this red-hot mass of air it self-ignites and the burning process continues again, until there is insufficient mixture capable of supporting combustion.

When the mixture was lit, the power stroke of the cycle began. Once again referring to Charles Law, as the now ignited mixture burns, its volume will increase with increasing temperature. The burning mixture causes temperatures in the cylinder to soar and now that the piston is on a downward stroke, the burning gases expand, forcing the piston downward at an increasing rate. Thus it is the simple expansion of heated gases that drive the piston and create...power. Ooo, ooo, ooo. (Thanks to Tim Allen for making this funny as well as productive.) This is a gross simplification of the combustion process which I regulate for a living but is understandable enough to suit this page.

And finally the Exhaust stroke begins when the piston has reached the lower-most point of travel and starts upward again. This time all the burned gases are forced out through the exhaust valve and away from the cylinder and the cycle can start again. Not much to this stuff is there?

The Hvid injection method is unique in that it uses no mechanical means to force the fuel into the combustion space. It is accomplished, like all good things, with an explosion. As previously noted, compressed air was often utilized as a means of forcing the fuel into the cylinder at the proper time. Two disadvantages should be immediately clear to the student (I've always wanted to say that). The first is the need to compress the air. The parasitic load required to do this is sufficient enough to warrant exclusion on smaller engines. The additional mechanical complication is not a desirable trait either. Less obvious is the chilling effect of the compressed air on the superheated air in the cylinder. Adding even a small amount of cooler air reduces both the ability of the mixture to ignite and the overall efficiency of the engine. Since the original intent of the compression ignition engine was to improve the efficiency, giving up any of the thermal advantage was a step backwards. The Brons Spraying Cup Method provided a way to inject fuel into the cylinder without the need for mechanical assistance. With the help of some drawings obtained from the patent applications, let's look at how this unique injection system works.