HEMI Engines Explained
January 12th, 2012 Muscle CarsHEMI Engines Explained
You may be surprised to learn that HEMIs were first designed in 1951. It’s only within the past decade that they’ve enjoyed a new surge of popularity (drivers today associate them with raw power). The name was derived by the shape of the cylinder heads, which are hemispherical (a departure from the flat-shaped heads commonly used at the time). In this article, I’ll describe the basics behind their design as well as their advantages and potential drawbacks.
Combustion Chamber Efficiency
First, I’ll explain the difference between heads that are hemispherical and those that are flat. In a flathead engine, a hole is drilled into the combustion chamber to allow the insertion of a spark plug. The valve and spring are typically positioned at the side of the chamber. It’s simplistic in design and comparatively less-expensive to make.
On the other hand, HEMIs use a round head for the combustion chamber with the spark plug inserted at the top. Twin valves and springs are also positioned on the top toward both sides (imagine a few inches above each ear on a person’s head). This design allowed for greater efficiency within the chamber.
Advantages
In any car engine, the main concerns involve maximizing thermal, energy, and fuel efficiency. You want your engine to burn as much gas as possible without wasting it. You also want to build as much pressure as possible within the combustion chamber to take advantage of the resulting energy. Finally, you want to minimize the loss of heat within the chamber.
HEMIs are designed with less surface area than flathead engines. Remember that greater surface area leads to greater loss of heat. As gas within the combustion chamber cools, it doesn’t burn efficiently. With a HEMI, surface area is limited, so heat is retained and pressure is maximized.
Potential Drawbacks
One of the disadvantages to this type of engine is an inherent limitation in their structural design. While they effectively limit surface area, it also makes it nearly impossible to have 4 valves on 1 cylinder. The reason this is a drawback is because cars that are designed with 4 small valves tend to allow the engine to breathe more easily. A second disadvantage is that HEMIs use large combustion chambers. That makes them susceptible to increased heat loss when compared to smaller chambers.
In the end, it’s a trade-off. HEMIs do provide more power to the driver and they’re priced accordingly. That said, many drivers will appreciate the efficiency of 4-valve cylinders. As technology continues to improve heat, pressure, and fuel efficiency in car engines, better options are likely to emerge in the near future.
Tags: Cars, HEMI Engines