Hemi is short for hemisphere. The term refers to a hemi engine which has been described as an internal combustion engine wherein the combustion chambers are of hemispherical form. For centuries hemispherical combustion chambers were used in cannon and mortars these chambers were initiated on some of the earliest automotive engines, prior to proving the concept of internal combustion engines. Since 1901 at any rate, hemispherical cylinders have been used in the following:
- 1905 by the Belgian car maker Pipe,
- 1912 by the Peugeot Grand Prix Car
- 1914 Alfa Romeo GO car
Stutz constructed four valve engines, theoretically anticipating modern car engines. The Toyota T engine, Peugeot 403 and BMW double push rod design are other well known hemi engines. Another notable example was the Harry Arminius Miller racing engines.
With the hemispherical combustion chamber design, the intake and exhaust valves are normally on opposite sides of the chamber which facilitates combustion mixture to flow directly across the chamber. This is generally referred to as cross – flow heads. Important challenges in the marketing and selling of hemi engines revolved around the design of the valve actuation, to improve its effectiveness, efficiency and reliability all at an agreed cost. Early in Chrysler’s development of their hemi engine in the 1950s, the head was referred to as the Double Rocker head in company advertisements.
Albeit a wedge – head design provides uncomplicated valve triggers, it generally required the fuel or air mixture to make sharp turns when it is going to and from the chamber. Larger valves are possible as well as a straighter, less restrictive flow path can be supplied for the fuel or air mixture, with a hemispherical chamber. This will impact on the engine breathing properly. Although it is not necessary, one can put the spark plug beside the center of the chamber aids to attain absolute combustion of the fuel or air mixture.
- There are however shortcomings to the hemispherical chamber these are as follows:
- Increase in production cost
- High relative weight – it is 25% heavier when compared to wedge head per Chrysler’s engineers
- Poor low – rpm performance qualities
- Complexities when meeting emission principles which have caused the hemi head to be unfavorable.
The Chrysler Corporation is by far the most widely known advocate of the hemispherical chamber design as the company used the trademark Hemi and has used it comprehensively in their advertising campaigns sometime in the 1960s.
Ford, Aston Martin, Porsche, Jaguar, Mitsubishi, Alpha Romeo, Lotus among others, have all produced and successfully utilized the hemi – head engines throughout the years. Several of the engines today utilize active combustion chambers that have been designed to swirl and tumble the fuel or air mix within the chamber for the most efficient combustion event possible. Today the hemi is not really a description of a combustion chamber; it is more so seen as a trademark.