Existing successful heat engines are steam turbines, gas turbines and positive displacement engines (reciprocating piston and rotary Wankel) utilizing various thermodynamic cycles (Diesel (or rather Sabathe), Otto and Stirling cycle). These engines, although now having been developed for more than century (almost 2 centuries in the case of Stirling), still stop short from fulfilling the requirements imposed on prime movers by modern economy. Thus steam turbines require huge steam boilers and steam condensers and are troublesome to exploit, therefore their applications are restricted to power plants and propulsion of ships and some other heavy machinery. Gas turbines, thermal efficiency of which can achieve even 65% in large units destined for power generation and industrial applications (e.g. in most recent large turbines built by GE, which in fact are compound heat machines with large heat exchanger), usually, particularly in small units, display much poorer figure than positive displacement engines, are more complicated technologically and more expensive, and therefore are unlikely to earn as dominant position as Diesels enjoy today due to these and other well-known inherent drawbacks and limitations. Thus positive displacement engines still have important advantages over turbines that render them irreplaceable for most applications.
Most common positive displacement engine in use (and in fact most common heat engine), Diesel engine, achieves maximum overall efficiency of slightly beyond 50% (large stationary or marine units, which again are compound heat machines comprising Diesel engine, turbocharger, supercharging air cooler and auxiliary power turbine), and average Diesel efficiency is merely ˜40%, a poor figure in comparison with 70-75% originally assumed by its inventor in late 19th century. Thermal efficiency of Diesel cycle rises with the compression ratio, but this method for improving overall efficiency of real Diesel engines is obstructed by friction loses rapidly rising with loads of engine's mechanism. Moreover, conventional connecting rod—crank mechanism's strength becomes a concern in highly loaded Diesel engines.
Another well-known positive displacement heat engine is the (external combustion) Stirling engine. This engine is closest to the ideal Carnot engine in terms of thermal efficiency, and another important advantage over known internal combustion engines is its capability to utilize various sources of thermal energy. However, Stirling engine is expensive to manufacture and troublesome to maintain, and this renders it considerably inferior to internal combustion engine in most applications, and prevents from earning wide acceptance.
There are many non-conventional designs of heat engines (most of them focusing on transforming gas force into driving torque of rotating shaft), e.g. rotary engines like Wankel, recently patented quasi turbine (see U.S. Pat. Nos. 6,164,263 and 6,899,075), spherical engines (see U.S. Pat. Nos. 6,325,038, and 6,941,900, and Russian patent 2,227,211) and oscillating pivotal engine (see www.PivotalEngine.com). However, so far none of those non-conventional engines, with Wankel-type engine being the only exception of economically (but certainly not conceptually) marginal importance, was successful, and probably none of them has any chance to even go beyond the stage of prototyping. Technically, this is due to the fact that the answer to the principal question any new engine is obliged to answer: “Does the new engine do its work better than conventional one?” is decidedly negative for all those non-conventional designs, including Wankel's. Even the answer to the more general question: “Does the new engine do its work in any aspect better than conventional one?” is negative for almost all non-conventional engines. (In the case of the Wankel engine, the answer to this more general question is positive, but superiority of Wankel over conventional engines in certain aspects (great power/weight and power/volume ratios, kinetic simplicity and smoothness of operation) is overshadowed by its inherent drawbacks (weak structure, inability to cope with large outputs, inferior efficiency, weakness of sealing, inherent inability to incorporate high compression ratios)). Conceptually, this is mainly due to the fact that those new engine designs (e.g. quasi turbine) focus on certain isolated aspects of heat engine while ignoring some other aspects (e.g. sealing, mechanical strength and reliability).
For example, recently patented positive displacement rotary engine, quasi-turbine, is complex both kinetically and structurally, its moving elements of complicated shapes are likely to be subjected to excessive thermal stresses and renders the engine weak structurally and more difficult to seal than Wankel engine; thus the engine is unlikely to do well the job of heat engine (it would be better as pump or compressor). Some other rotary engines (e.g. satellite engine, see publication WO9618024) use toothed wheels to transfer the pistons movement to rotary motion of engine's shaft. This not only makes these engines complex but also unreliable, as engine's elements that meet along a line are not well suited to bear shock loads met with in internal combustion engines.
Fuel cell is a very promising source of power for many applications, but it seems improbable it will become appropriate for applications where high power density is essential in any foreseeable future.
Thus there is a need for highly efficient universal source of mechanical power, and highly efficient and clean thermodynamic processes for producing hot high pressure gases, like detonation, compression ignited combustion of homogeneous charge and very high-pressure Sabathe cycle, render positive displacement internal combustion engines a very interesting proposition, provided that efficient way for converting thermal energy into useful mechanical power is incorporated. It is to be stressed that lack of such effective method for converting thermal energy into driving torque is an important obstacle to develop a practical Homogeneous Charge Compression Ignition (HCCI) and Positive Displacement Detonation (PDD) engine. The reason is that maximum gas forces themselves, as well as gradients of gas forces (understood as function of time), met with in HCCI and PDD engines (at least those utilizing stoichiometric mixture, which is the most efficient thermodynamically, and also most efficient from the point of view of power/weight and power/volume parameters) are much higher than in conventional IC engines, and conventional mechanisms are unable to cope with such extreme loads. This is one of the reasons, for which the planned “HCCI engines” are to utilize the more efficient HCCI mode of operation only while producing power at a moderate rate (and working on loan mixtures), converting into ordinary Diesel mode of operation when the power demand rises (the other reason is that IC engine working on loan mixture produces less pollutant nitrogen oxides).
It is to be stressed that none of the non-conventional engine designs in United States Patent and Trademark Office (USPTO) and European Patent Office (EPO) patent data bases offers satisfactory mechanical structure of the ICE suitable for coping with extreme loads while assuring engine's compactness and good sealing. Moreover, none of the known positive-displacement internal combustion engines approaches highly desirable structural simplicity of gas turbines.