Abstract "Hybrid" is defined as "anything of heterogeneous origin or composition" and "derived from unlike sources." Con- sidering reciprocating, piston-type engines as an example, engineering features characteristic of the otto and diesel en- gines are well known and are significantly different. When one or more features of the otto engine are combined with or replace significant features of the diesel engine, the re- sult is a "hybrid." The major objective of such hybridiza- tion today is to attempt to achieve the improved low-load fuel economy of the compression ignition engine without sacrificing lightness, smoothness, quietness, and clear exhaust of the spark ignition, carbureted engine. IT IS NOT UNCOMMON in the history of thought that an- tagonistic or seemingly antagonistic theories merge. This occurs quite often in the fields of philosophy, sociology, and religion. Limiting our thinking to the exact sciences, we may cite the particle and wave theories of light as an ex- ample. Internal combustion engines have evolved for over 60 years along two distinctly separate lines: the otto engine and the diesel engine. Those features of the diesel engine which are significant of the compression ignition principle are the injector, a very high compression ratio, and absence of an air throttle (see Table 1). Each of these features, or components, performs a defin- ite function. Characteristic functions of the otto engine and the diesel engine are shown in Table 2. It is readily admitted that these combinations of func- tions are logical, not merely incidental. For the otto en- gine are: 1. External proportioning and mixing of the air and fuel Table 1 - Characteristic Features Otto Engine Diesel Engine Carburetor Spark plug Intake throttleInjector Very high comp. ratio No throttle Table 2 - Characteristic Functions Otto Engine Diesel Engine 1. External mixing 2. Homogeneous 3. Fixed point ignition 4. Air + fuel regulation (Throttled Mixture)A. Internal mixing B. Heterogeneous C. Compression ignition D. Fuel regulation (Unthrottled Air)Hybrid Engines Paul H. Schweitzer Schweitzer & Hussmann Lawrence J. Grunder Richfield Oil Corp. is desirable to obtain a more homogeneous mixture within the cylinder. 2. A homogeneous mixture is desirable because chem- ical reaction can take place only between adjacent oxygen and hydrocarbon molecules. 3. A precisely timed electric spark is an effective means for initiating controlled ignition at a single peint. 4. Quantity regulation in the form of an intake throttle had to be accepted in view of the relatively narrow range of ignitable air-fuel ratios. In the diesel column at the right in Table 2, We will start at the bottom with: D. Fuel Regulation. At all loads and speeds, the air is unthrottled -- only the fuel quantity is varied by regulation. In this way increased air pumping losses at light loads are avoided, and, in addition, there are smaller heat losses be- cause of lower flame temperatures. C. Compression ignition becomes a necessity in order to ignite a wide range of mixtures, and this is accomplished through raising the compression ratio above 14:1. B. Heterogeneous mixture provides a variety of local air- fuel ratios insuring the burning of lean overall mixtures by initiating ignition wherever the air-fuel mixture ratio is just right. A. Internal mixing of the fuel with the air is permissible as perfect mixing is not necessary. The success achieved by both the classical otto and the classical diesel engines cannot be denied, and is due, no doubt, to the logical pattern followed in the selection of their characteristics. Nevertheless, significant advanceshave been recorded recently by mixing otto and diesel charac- teristics, and more and more will be heard of such "hybrid" engines in the near future. Hybrid combinations total 14 and are shown in Table 3. Some of them are clearly impossible or impracticable. There are, however, quite a few of the hybrids which have achieved considerable