22 Past and Current Trends in Engine Development

Natural gas engines have been used for power generation since the mid-1940s, and have evolved steadily. The earliest gas engines were derived from diesel blocks and incorporated many of the same components as diesel engines. Spark plugs and carburetors replaced fuel injectors, magnetos replaced fuel pumps, and lower compression-ratio pistons were substituted to run the engines on gaseous fuels. The first gas engines were set to run at optimum power output without regard to emissions and fuel efficiency. Where fuel was plentiful and could be delivered at little or no cost (often straight from the gas well to the engine fuel inlet), these engines were economical for producing local power for electric generation or for driving pumps and compressors. Their mechanical efficiency was about 25 percent. Most of these engines were naturally aspirated — the absence of turbocharg-ers and charge air aftercoolers made them simple to apply and maintain. However, power levels could not reach above 100 psi brake mean effective pressure, while diesel versions could achieve more than 200 psi. (Brake mean effective pressure, or BMEP, is a measure of engine power output because HP = BMEP x piston area x stroke x speed.)

Since the mid-1980s, gas engine manufacturers have faced growing pressure to increase fuel economy while lowering NOx emissions. Leaner air/fuel mixtures requiring turbochargers and charge air coolers were used. The leaner fuel mixtures and lower in-cylinder firing temperatures sharply reduced NOx from about 20 to below 5 g/bhp-hr. Lower cylinder temperatures also meant that BMEP (and, thus, power output) could increase without the damaging effects of hotter exhaust gases on valves and manifolds.

Lean-burn designs, however, have drawbacks. Fuel economy decreases because of turbo pumping losses and less efficient lower-temperature combustion. Additionally, the risk of detonation increases. Detonation, a premature explosive combustion of fuel, can severely damage pistons, liners, and cylinder heads. In response to this, manufacturers began to install control systems, including detonation sensors, that automatically retard ignition timing. Manufacturers also began to install swirl plates and high "squish" pistons to increase turbulence in the cylinders at spark ignition. The result is more complete combustion and greater efficiency. Other advances include solid-state ignition controls to improve timing accuracy and electronic controllers that automatically maintain the optimum air/fuel ratio, adjusting for changes in air temperature and pressure, fuel heating value, and other operating variables.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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