Benefits of LIM Technology’s proprietary systems for Compression Ignition Engines

In experiments and prototypes dating back to 2000, LIM Technology has demonstrated that compression ignition engines can operate efficiently in the configuration described below.

Such an engine will be configured with exhaust ports in the cylinder liner, distributed about its circumference just above the position of the piston crown at bottom dead center. This enables the piston to act as sole exhaust valve. Using the piston as the sole exhaust valve allows all valves in the cylinder head to be used for intake only.

The originality of this concept is not LIM’s. It has been done before and worked well, according to Rodi Power Systems, a company which did not survive, but who had built and tested such an arrangement.

What is different is the management of the “intake” valves, which are located in the cylinder head. We use quotation marks around the word “intake” because the air is more pushed than it is pulled. Thus, it is not “intake”, but “input”. Nonetheless, in order to conform with normal practice in the internal combustion industry, and for convenience, we will call these valves “intake”, hereinafter, without quotation marks.

The core technology created by LIM is the simple intake valve.

Since at least 1937, uniflow two-stroke diesels have been manufactured and marketed by Detroit Diesel, and possibly others. These ”uniflow” engines use ports as described above, but the ports are used to allow air to be forced into the cylinder. In such engines, exhaust is released by mechanically managed valves. These valves are conventional poppet valves held closed by stiff springs and opened by a conventional valve train with conventional mechanical parts. Thus, “Uniflow” has come to be used to describe a system whose intake air enters at the bottom, with exhaust leaving at the top.

Conventional valve trains have expensive cam shafts as their essential timing devices and such a cam shaft is driven either by belt, chain, or gears so that it is always actuating the valves at the precisely engineered time. This is the dominant conventional system. Conventional valve trains were probably first devised for four stroke engines in the 1870s.

Both four stroke and two stroke engines can be either spark or compression ignition. Certain classes of users prefer either one or the other, but for maximum fuel efficiency, compression ignition is preferable.

Having stated that Rodi Power Systems had a reverse uniflow system, with air entering at top and exhaust leaving at bottom, then we should consider what possible advantages come from such a system. The most obvious is the coolness in the head. With fresh air entering all cylinder head valves, and exhaust leaving via the ports at the bottom, there is much less heating of the head. This may also make possible less propagation of NOx. Another benefit will likely be a smaller cooling requirement.

LIM’s principal improvement is therefore not the cycle itself, but the simplification of the valve train. In a LIM (reverse uniflow) engine, the intake valves are powered open by the difference in air pressure. The valves themselves are of a different shape from, and can be less than two thirds the mass of, any comparable conventional valve of the same material.

Even in a conventional valve train, this weight saving would reduce energy expenditure in valve management. In the LIM valve management system, however, there is no other mechanical actuation except the system to pressurize the intake air plenum.

Conventional valve trains use heavier valves for many reasons. The forces used to open a valve against its stiff spring are huge, and the mechanical parts other than the valves have considerable mass, which must be moved non rhythmically, requiring significant energy. The movement of mass for each conventional valve opening event would seem to be twice or more than that of such an event in a LIM type system.

Closing the valve is theoretically possible without mechanical actuation, as the early stage of the compression portion of the cycle brings cylinder pressure to more than intake plenum pressure. In the low boost pre-prototypes so far constructed, this has not, so far, been effective.

In a LIM type engine, other than the spring and intake air pressure, there is no mechanical management of the valve. One possible detraction from efficiency, according to certain “knowledgeable” people would be valve bounce. In our early testing, we had no means to analyze this, but we consistently found sufficient compression for ignition to occur. It seemed that valve seating was both timely and effective, as long as the spring tension was adequate.

Very early in LIM’s experiments, light valve springs were attached to the valves to keep them in the closed position. It was soon discovered that enhancing valve management to harden the spring at higher engine speeds enhanced performance. Spring force need not vary during the cycle, and is non-cyclic in the current stage of development. Intra-cyclic spring manipulation has not been tried, but may add considerable value.

Although three prototypes are currently available for demonstration, none is a fully LIM configured design.

Theoretically, given a very high pressurization of incoming air and unobstructed exhaust, it seems possible that no mechanical or other valve closing mechanism may be required. Some of the time, however, during starting, idling, or other periods of low output requirement, high pressurization may not be desirable or efficient, so it seems likely that valve management of a more deliberate nature will continue to prove valuable.

Two valve closing systems have been tried that seem to have valuable qualities. The simple spring system with a non-cyclical force variability has worked consistently. It has some drawbacks, but these are not so serious as to rule out continued use of, and enhancement of, the variable force spring system, which is patented.

An obvious concern regarding spring management is that at higher rpm, to induce timely closing, the spring force may impede opening and thereby shorten dwell. This has proven to be a constraint, but not so serious as to negate the value of the system. An alternate system has been developed and is patented.

The alternate system is to use pressurized air delivered when needed, to force the valve to begin closing at the right time. In a reverse uniflow engine, this could be managed either electronically, although the appropriate electro-mechanical hardware seems not to be commercially available at this time, or mechanically, but with far less friction and inertial losses than a conventional valve train. One demonstrator of this technology exists. It enables higher speed operation than the spring system previously installed.

Questions raised by knowledgeable people regarding valve bounce were answered in 2006, with the publication of a report by Douglas M. Baker, Ph.D. Dr. Baker describes how valve movement was measured at high speed. Bounce does indeed occur, but it is far less than serious. Time to open the valve was faster, to full lift, dwell was longer at full lift, and closing was faster to full rest than normal camshaft design allows. What was lost to valve bounce was far less than what was gained by the overall efficiency of the system.

Because the cylinder head valves are used only for intake, they are generally cooler. Because the head is never involved with exhaust, it stays comparatively cooler than in a conventional engine. This is not the case in a conventional uniflow diesel with exhaust through its head.

Intake valves do not accrete soot as exhaust valves do. Probably because they remain comparatively clean, the intake valves consistently seat well.

LIM’s novel system offers outstanding valve opening timing at every rpm, ungoverned by fixed timing of a cam shaft. This means that at start-up, air is trapped effectively enough on the first revolution to allow fully effective compression and thereby allow prompt starting. Prompt cold starting is a major benefit of the system.

The valves are allowed ample clearance, and move easily in their patented guides, allowing for the elimination of liquid lubrication. This means that no oil need circulate in the head. Since there are no fluids required for valve management, there are no seals and therefore another source of friction is eliminated. This also simplifies construction of the lubrication system.

In a LIM Simple Cycle engine, there is in fact no lubrication except in the crank case and at the piston rings. There may be a lubrication system in the turbo charger or super charger, but lubricant need not circulate in any other engine systems.

Advantages of this system in applications which are weight critical are several. Easier starting should enable lighter starting equipment. Less need for lubrication indicates a smaller oil pump; less need to cool the head indicates less coolant and a smaller coolant pump. Elimination of virtually all conventional valve train parts certainly contributes to weight and cost reduction.

Crank case oil is kept in the crank case except for that miniscule amount used to lubricate the cylinder liner and piston rings. To keep the oil where it belongs, the piston skirt in a LIM type engine is slightly longer than in non-uniflow engines, with a second set of rings near the bottom of the skirt. These rings remain below the exhaust ports. They deposit some oil below the exhaust ports which is then taken by the lowermost upper ring.

New methods of honing or coating the cylinder wall inner surface may reduce the need for cylinder wall lubrication; such new systems are commercially available now, but have not yet been tested in a LIM type engine.

Compression ignition, heavy fuel engines are consistently more fuel efficient than comparable spark ignition engines. Good power for weight, and compression ignition fuel efficiency make a LIM type system a good choice in many military, aviation, marine and automotive uses.

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