It takes energy to compress an air/fuel charge, spin the individual piston through its four strokes and overcome the friction inherent in all the moving parts. This is negative energy, or the amount of energy it takes to produce the mechanical work needed in the combustion process. The energy released in combustion is positive energy. The overall efficiency of the engine, then, is the difference between these two, called net energy.
Today’s internal combustion engines are relatively unchanged from the original designs used in the very first automobiles. Yes, they have been improved upon as the science of building them has moved forward. Yet, they are still relatively inefficient in that there is still too little net energy left to operate the car. Many of the changes we’ve witnessed over the last several years are aimed at, and have been successful in, improving that efficiency. But there is still a lot of room for improvement.
Some of that improvement has come from revisiting old ideas that just weren’t feasible when first developed. But with modern technology being what it is, some of these old ideas have new life, and may just show up in production in the near future. Let’s take a look in our crystal ball at two designs that have recently made the news.
The Scuderi Engine
The Scuderi Split Cycle engine made its debut as a “proof of concept” prototype at the 2009 SAE World Congress in April. It is a 1.0-liter, 2-cylinder, naturally aspirated gasoline engine. It is the brainchild of Carmelo Scuderi, a thermodynamics and fluid mechanics engineer born in 1925. Scuderi often considered how to improve the efficiency of existing Otto Cycle internal combustion engines and began work full time on his design in 1998. He passed away in 2002, but his work is continued by his family and the Scuderi Group.
The split cycle design concept is not new, with designs recorded as early as 1914. A split cycle engine “splits” the functions of the conventional 4-stroke by using one piston strictly for intake and compression, and a paired piston for power and exhaust. This design allows the power piston to fire every cycle, instead of every other. The first designs were extremely inefficient for two main reasons: first, poor volumetric efficiency (breathing) caused by high pressure gas remaining in the compression cylinder; and second, low thermal efficiency caused by igniting the mixture conventionally, before top dead center. The Scuderi design addresses both of these concerns.
Volumetric efficiency is improved by bringing the compression piston to within 1mm of the cylinder head. This requires valves that open outward, rather than inward, as in a conventional design. The intake, exhaust and crossover valves are all controlled pneumatically using air pressure generated by the engine itself. They also are fully variable and are used to control engine load instead of a conventional throttle plate. Thermal efficiency is improved by firing the air/fuel mixture 15 degrees AFTER top dead center — a novel concept that is possible due to the high pressure air charge delivered by the compression piston through the crossover passage. Fuel is delivered by direct injection, using injectors especially designed by Bosch for this application.
Here is how it works: Air is drawn in on the intake stroke by the compression piston. On conventional engines, compression ratios are limited by the combustion process. The Scuderi Split Cycle design has no such limitations, because the compression piston is separate from the power piston, so a compression ratio of 100:1 is used to squeeze the air and generate pressures near 735 psi.
This high pressure air charge is then passed to the power piston side through a crossover passage. The two pistons are slightly offset in their travel, and the high pressure charge entering the power side creates a lot of turbulence. Combined with a direct injection of the fuel at nearly 2,900 psi, the result is rapid atomization of the fuel and faster burn times, allowing the actual ignition to occur after top dead center. The power piston then expels the remains through the exhaust valve.
The prototype is vastly more efficient than earlier split cycle designs, but is on par only with current 4 stroke engines. The promise, though, is in the addition of turbocharging and/or the use of the design as an “air” hybrid. The compression side is nothing more than an air compressor, and that air can be stored.
This would open up a whole realm of possibilities like “idle off” mode, or disabling the compression side and utilizing the stored air charge instead. Initial forecasts predict fuel economy with this design equal to today’s electric hybrids, and could even be paired with most of today’s electric drives, replacing the Internal Combustion Engine (ICE) now in use and increasing fuel efficiency even further.
For more information on this engine, visit www.scuderiengine.com.
The MCE-5 Variable Compression Engine
Currently, engineers can vary many of the running parameters of the engine. Valve timing, valve lift and intake length are just a few of the examples of technology already in production. But what about varying compression? As mentioned earlier, the energy expended to compress the air/fuel charge is a negative — being able to alter that while still meeting driver demand could improve efficiency.
The idea of a variable compression ratio (VCR) is not new either. In March 2000, Saab unveiled a prototype 1.6 liter engine equipped with VCR called SVC, or Saab Variable Compression. The engine delivered 228 hp and 225 foot-pounds of torque, with a 30 percent reduction in fuel use as compared to a conventional engine of similar power output. A few months later, FEV Motorentechnik displayed an Audi A6 1.8 liter VCR engine that produced the same power as a 3.0 liter, while reducing fuel use by 27 percent.
The MCE-5 design was invented by Vianney Rabhi, head of strategy and development for the MCE-5 Development company. The design is called VCRi, or Variable Compression Ratio, intelligent. It is capable of providing varying compression ratios from 7.0:1 to 20:0:1 and can do so per cylinder, independently of the others. Reaction times are less than 100 milliseconds from minimum to maximum, with an accuracy of 0.1 mm. A 1,484 cc version installed in a Peugeot 407 was recently displayed in Geneva. Equipped with two-stage turbocharging and variable valve timing, the engine produces 217 hp and 310 foot-pounds of torque.
According to Rabhi as quoted in the SAE publication Automotive Engineering International, the system is based on a gear transmission system. The connecting rod acts like a fulcrum, and a control jack, controlled by oil pressure, is used to alter the position of the cylinder piston relative to the cylinder head.
The separate cylinder piston is joined to the control rack by the gear wheel. The motion of the cylinder piston is guided by a synchronized roller, eliminating loads caused by rod thrust and piston slap and promising longer overall service life. The rod/crank relationship remains unchanged, with Top Dead Center (TDC) remaining the same regardless of the compression ratio being applied. The overall design requires only a few additional components, and the company claims the concept can be easily incorporated into existing engine designs.
For more information on the MCE-5 VCRi, visit www.mce-5.com.
Food For Thought
As I write all this, President Obama is holding a Town Hall meeting being broadcast on TV. The topic at this moment is global warming and other environmental issues. It is clear that we are moving in a direction of regulated environmental standards that will impact our businesses in a number of areas. And that may be a good thing for our children and their children.
At the very least, making our cars more efficient will save resources and reduce that environmental impact. These two designs are just a few of those being developed today, and if history is any judge, they will be the designs we fix in the future.