The document describes an innovative opposed piston engine concept called the Paul Engine that could achieve unprecedented levels of fuel efficiency and power density. Key aspects include using opposed pistons without a cylinder head to improve efficiency, utilizing high compression ratios and supercharging, and incorporating heat recovery from the exhaust and cooling systems. Experimental prototypes demonstrated efficiencies over 50% and power densities up to 600 kW/L. Further developments aim to adapt the concept for automotive and other applications through modifications like a four-stroke cycle and integrating heat recovery into a compact design. The innovations have the potential to revolutionize engine technology by substantially reducing fuel consumption, emissions, weight, and cost.

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Ultimate IC engine technology based on Paul Mechanism
Liviu Giurca
INTRODUCTION
The Paul Engine was patented by Dr. Marius Paul, an American engineer having Romanian origin. In Romania he was the director of National Thermal Engine Institute from Bucharest (a very prestigious institution designing tank, marine and locomotive engines). After his arrival in USA, Dr. Paul created a company named Engine Corporation of America which began to develop the first variants of the Paul Engine. The US Army was immediately interested by his concept and funded an intensive development. The results were so impressive than the concept remained top secret for 20 years. Now his patents are available for civil development.
Mainly the Paul engine uses an innovative arrangement of the connecting rod – crankshaft mechanism and can be achieved with cylinder head or having opposite-pistons, without cylinder head.
STATE OF THE ART
Historically, the opposed-piston engine set combined records for fuel efficiency and power density that have yet to be met by any other engine type. The fundamental thermal efficiency benefits of this engine type along with its low emissions, small package size and weight, and low cost relative to current engines make it an attractive alternative for future commercial and passenger vehicles. The precursor of this concept was the Junkers Jumo 207 Diesel Engine.
Opposite piston Engine: Unmatched combination of Fuel Efficiency and Power Density
http://en.wikipedia.org/wiki/File:Opposite_piston_engine_anim.gif

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Some companies already developed concepts which present high power density. It is the case of the OPOC (opposed piston opposed cylinder) engine presented by Ecomotors with FEV.
OPOC - Ecomotors
It comprises two opposing cylinders per module, with a crankshaft between them, each cylinder having two pistons moving in opposite directions. This innovative design configuration eliminates the cylinder-head and valve-train components of conventional engines, offering an efficient and simple core engine structure. The result is an engine family that is lighter, more efficient and economical, with lower exhaust emissions. The big advantages of this concept are the power density which is around 2 kW/kg and the effective efficiency which is at least 37% in real proved conditions. The disadvantages are: -It is not adapted for automotive application, the external volume of the engine being developed into a single direction; -It is designed as a single unit with two cylinders and to have more than that (4, 6 or 8) is very complicate; -Not four-stroke solution proposed.
Another configuration was made by Achates Power.
Achates Power Diesel engine

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It comprises two opposing pistons per module, with two lateral crankshafts, each cylinder having two pistons moving in opposite directions. This innovative design configuration eliminates the cylinder- head and valve-train components of conventional engines, offering an efficient engine structure. In conjunction with the thermal efficiency advantage inherent to opposed-piston engines, this concept realizes significant reductions in fuel consumption over conventional four-stroke compression ignition engines. The disadvantages are: -The scavenging of the cylinder is made by an external compressor, electrically acted, which present an important cost. - It has a complex construction. - Not four-stroke solution proposed.
PAUL ENGINE – THE MOST EFFICIENT ENGINE EVER BUILT
The engine concept is based fundamentally on thermodynamic laws, which provide an undeniable solid basis for this development. The main thrust of this new engine concept is to produce the highest pressure thermal cycle possible. The pressure achieved are well above those obtained in current state-of-the-art of 120 to 180 daN/cm². The engine concept discussed here is pushing pressures to the new frontiers: 300 to 350 daN/cm². Consequently it employs real compression ratio between 30 to 60:1 (with high supercharging ratio) and ideal cycle efficiency of:
In the last time, few experimental works, done by some independent prestigous research entities, demonstrated in paralel this posibility for compression ignition engine [4]:

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And for spark ignition engines [5]:
These digrams from [4] and [5] don’t take into account the heat recovery from the exhaust gases and from the cooling system.
To attain these goals it is necessary to overcome basic conceptual design and structural barriers that have blocked further advances in engine technology for the past 100 years.
The main barriers that have been eliminated are: 1. The cylinder head, valves and valve mechanism ; 2. The side force between piston and cylinder liners; 3. The piston rings with high back pressure; 4. The truncated expansion stroke; 5. The heat losses.
The Paul concept is based on structural and fundamental principles which are: 1. Opposed piston engine – without cylinder head, gasket, valves, etc. 2. Ultra-high compression ratio and supercharger level; 2. Counter rotating mechanism, eliminating the side force and improving engine balance; 3. Piston rings without back pressure; 4. Extended expansion stroke using in the most efficient manner the combustion gas energy; 5. Symmetrical active force clamping the block and the opposed crankcases, pre-compressed by longitudinal bolts. By pre-loading the engine, the pre-compressed structure of the block and the crankcases are relaxed during combustion, creating an enormous capacity for high peak pressure and high power density.
This advanced engine technology was demonstrated by Engine Corporation of America, the project being sponsored by the U.S. Department of Defense through the Defense Advanced Research Project Agency (DARPA), implicating Sandia, Los Alamos and Lawrence Livermore National Laboratories and a group of U.S. Universities.

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The key enabler for a dual crankshaft engine with a constrained piston movement by two con-rods with theoretically no piston side forces is provision for forgiveness towards tolerances, which can lead to off-design positions of the piston in its cylinder bore and unfavorable mechanical effects such as scuffing, sticking or simply higher friction as the least bad of effects.
A simpler variant of Paul engine was also evaluated.

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The demonstrated engine advances that result from this research are: 1. Highest power density between 300 to 600 kW/l; 2. Reduced CO2 level; 3. Compensation of all rotating inertial forces; 4. Compensation of all 1st order oscillating forces (torque compensation); 5. No reaction due to torque impulses on the exterior of the engine are detectable; 6.Very low friction losses from the guidance of the piston by the connecting rods (the lateral guiding force of the piston is effected by the connected rods); 7. The use of large swept volumes with better thermodynamic efficiency; 8. Lowest fuel consumption between 95 and 125 g/kWh; 9. A dramatic reduction in cost of production and exploitation; 10. Universal use, i.e., military or commercial.
The recently demonstrated values have established new frontiers that are now opening the possibility of achieving enormous increases in power density and efficiency in future engines. An early operational prototype of Paul engine is shown below:
Almost 20 years ago (in 1996), this variant already obtained 52% Brake Mean Efficiency and a huge power density, without heat recovery,
Some versions of Paul Engine are successfully used to capture and to re-distribute the wind/solar renewable energies or even nuclear energy. One of these applications works in space from 20 years.
FIRST CIVIL EXPLOITATIONS OF PAUL MECHANISM
Due to their high efficiency, low weight and economic fuel consumption as well as their unbeatably low vibration, a German company NEANDER began to build multi-cylinder or mono-cylinder four- stroke Diesel engine using Paul mechanism, considered ideal for many leisure and commercial applications [3]. FEV North America, Inc. a leading developer of advanced powertrain and vehicle system technologies, which collaborated in the past with Dr. Paul, showcased a turbo-diesel outboard marine engine developed in cooperation with Neander Motors AG at the SAE World Congress 8-10 April at COBO Center in Detroit.

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http://www.neander-motors.com/
FURTHER DEVELOPMENTS
Dr. Paul mechanism is even more improved, for conventional or hybrid automotive applications as well as for stationary units, using the ideas of the author of this presentation: eng. Liviu Giurca. This evolution means a new engine technology named SE (Supercharger Engine), having the following benefits:
• Also four stroke operation
• Also spark ignition operation
• Low system cost
• Low weight
• Ultra compact and small
• Silent
• Low vibration
• Easy to integrate
• Emission compliant (incl. CO2)
• Multi fuel capability
• Heat recovery from exhaust system integrated in the engine volume
• Simultaneously, heat recovery from cooling system
In the table below can be compared the estimated efficiency for different SE solutions in the configuration as spark ignition (SI) and compression ignition (CI) engine.
No.
SE Variant
SI
CI
1
Four-stroke/Two-stroke highly supercharged without heat recovery
40 % – 45 %
50 % – 55%
2
With Exhaust gas heat recovery
50 % – 55%
60 % – 65%
3
With Exhaust gas heat recovery + Cooling system heat recovery
60 % – 65%
70 % – 75%
The estimated SE global efficiency

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Modern late 21th century power plants, utilizing combined heat and power, could yield an overall efficiency as high as 60%, where different working fluids drive different power driving cycles, obtaining a combined thermal efficiency. In the present case both, the produced energy and the recovered energies are collected as power directly by the engine shaft and all the process is developed in an ultra-compact and simple structure.
The SE technology can be considered as “revolutionary” in terms of advantages and as “evolutionary” in terms of engine modifications (one crankshaft more).
Mr. Giurca improvements are patent pending.
Author e-mail: lgiurca@hybrid-engine-hope.com
Bibliography:
1. CALSTART - DEFENSE ADVANCED RESEARCH PROJECTS AGENCY COOPERATIVE AGREEMENT MDA972-95-2-0011, QUARTERLY REPORT, July 1 to September 30,1997
2. Advanced Components for Electric Vehicles and Hybrids Electric Vehicles, NIST Workshop, October 27-28, 1993, Gaithersburg, Maryland 3. http://www.neander-motors.com/
4. Chris F. Edwards& others, Remarks on the Efficiency Potential of Chemical Engines, Stanford University
5. http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node26.html