Air Density: Are Boost Gauges Fooling Us?

One of the things I enjoy the most about this job is being able to spend time with and learn from the industry’s greats. People such as Gale Banks for example are so influential in the diesel world and it’s because of these industry icons that we have such a healthy diesel industry.

Speaking of Mr.Banks, I’m lucky that his facility is so close to me and it’s because of this that I get to talk with him in person fairly often. This guy will make the smartest of diesel guru’s feel stupid from all the knowledge he has. I’ve been a diesel guy from a very early age; I grew up around large marine diesel engines, worked on them at a very early age and got my first diesel pickup at age 21. For the most part I’ve been the only one to work on it. I like to think I have a pretty decent grasp on how diesel’s work and how to make them perform better. But every time I go see Gale it amazes me how complex diesel performance really is, and how much more I have to learn. The man is an internal combustion and compression ignition genius.

During my last trip to the Banks compound the topic of conversation was air density. Now we all know fuel needs air to burn. And we know that cool air is denser, therefore allowing it to hold more fuel (and burn more fuel, efficiently). So the cooler the air charge is the better (which is why most of us have massive intercoolers on our trucks). Gale explained that the modern boost gauge really doesn’t give us as much info as we think it does. We tend to think that more boost is always better. But if that air is extremely hot, it will be less dense, hinder combustion and hurt performance.

Think about it this way; if you take a can of compressed air and heat it up it will eventually get so hot that the pressure inside rises to the point where the canister can no longer contain the pressure. The can will then burst to relieve the pressure. Gale showed me another example of this by heating up a sealed steel container with a pressure gauge attached to it. At ambient temperature the pressure gauge showed zero pressure. Once he had turned the heat on (using a simple hot plate like the one you may have in your kitchen) the temperature and pressure within the sealed container began to rise. After 30 minutes or so there was over 3 psi on the gauge. There was no more air in the container than it started with, it was sealed, and yes I checked. As air warms it expands. At the same time when air is compressed it heats up. So when our turbos compress air they are also heating it up creating even more pressure, but without actually adding more air to match. This is just physics. As air is compressed it heats up, and therefore expands thus creating even more pressure. But pressure created by heat does not mean there is more air to help burn more fuel. Here’s where the boost gauge may be fooling us.

The boost gauge is a simple device that only measures air pressure, hot or not. Seems simple, and it is. Our boost gauges will not tell us how much air is available to help burn fuel. So for two trucks with the exact same engine setup, but one with an intercooler and one without, you will see higher boost pressure on the one without an intercooler. When you think about it we’ve been seeing a ton more water-to-air intercoolers for exactly this reason. They, when setup correctly, are much more efficient at scrubbing heat from an air charge than a traditional air-to-air intercooler is. Long story short, 100psi on your boost gauge does not mean exactly what you may have previously thought. I’ll be adding a ton of sensors to my 7.3L to help us better understand what individual performance parts do for air density (as well as many other things). I’m curious to see what pressure and temperature sensors before and after the intercooler will show me. At the same time Gale Banks has been working on a density sensor and gauge setup that will tell us exactly how dense the compressed air entering our engines is. It’s simple technology but some that I’m excited to play with and to see where it takes diesel performance to. DW