Compression ignition comes to gas engines

What many consider the Holy Grail of combustion technology is one step closer to prime time.

In the race to field energy-efficient vehicles, don’t count out internal combustion technology just yet. GM recently debuted its homogeneous-charge-compression-ignition (HCCI) system in two drivable concept cars, a production-based 2007 Saturn Aura and Opel Vectra.

“HCCI was a dream of engine designers when I was an engineering student years ago,” says Tom Stephens, group vice president, GM Powertrain and Quality. “Today, using mathbased predictive analysis and other tools, we are beginning to make this technology real.“

HCCI is the capstone of an integrated suite of engine technologies that includes central direct-fuel injection, variable-valve lift, mechanical camshaft phasing, and individual cylinder pressure sensing. HCCI engines are said to use 15% less gas than conventional port-fuel injected engines, and meet current emissions standards.

Unlike spark-ignition gas engines or compression-ignition diesel engines that have a combustion process characterized by growth of a flame front from a single point in the combustion chamber, HCCI produces a flameless, simultaneous release of energy throughout the entire combustion chamber. Lack of a flame and hot zones lowers combustion temperature and NOx emissions.

Fuel-air mixtures are comparatively lean, which helps the engine approach the efficiency of a diesel, but without the need for costly lean-NOx after-treatment systems. Burning less fuel at lower temperatures also cuts the amount of heat energy lost during combustion, boosting efficiency. HCCI engines have a compression ratio of 12:1 (similar to that of a conventional direct-injected gas engine), so they can run on regular pump gas and E85. GM says HCCI engines will cost less to build than diesels because the latter need stronger components to withstand compression ratios greater than 20:1.

“Perhaps the biggest challenge of HCCI is controlling the combustion process,” says Uwe Grebe, executive director for GM Powertrain Advanced Engineering. “With spark ignition, you can adjust the timing and intensity of the spark. But with HCCI’s flameless combustion, you must change the mixture composition and temperature in a complex and timely manner to get comparable performance.”

Having ample heat in the combustion chambers is key to making HCCI work. The engines use a conventional spark ignition for cold starts and when HCCI is disengaged. Fuel comes from conventional injectors located in the center of each combustion chamber. A controller uses special algorithms and feedback from the cylinder pressure sensors to adjust cam timing and fuel injection in the milliseconds between combustion events. “Going to HCCI mode from conventional spark ignition signals the fast-mechanical cam phasers and a variable-valve-lift mechanism to close the exhaust valves early in the exhaust stroke, trapping some of the hot residual combustion gases in the combustion chamber,” explains GM Global HCCI Program Manager Matthias Alt. “This helps maintain a high cylinder temperature to facilitate auto-ignition when the fresh airfuel charge is added next cycle.” Operation at cold ambient temperatures necessitates trapping more hot gas in the combustion chamber (earlier exhaust-valve closing), for example.

Currently, the GM demonstration prototypes can run in HCCI mode to about 55 mph, going to spark ignition at higher vehicle speeds and under heavy engine load. A goal of the program is to extend HCCI’s operating envelope through refinements to the control system and engine hardware. GM says HCCI will work on any gasoline engine in its inventory and could combine with hybrid technology. No release date has been set for production HCCI-engine cars.

 HCCI first drive

Machine Design Editors Lawrence Kren and Robert Repas recently drove an HCCI-engine Opel Vectra and Saturn Aura at GM’s Milford Proving Grounds in Milford, Mich. Driving the cars at modest speeds and accelerations automatically engages HCCI mode, accompanied by a diesel-like clatter from the engine. The clatter was less pronounced in the Opel Vectra, however. GM engineers credit a special diesel noise-abatement package in the European-spec Vectra with the lower cockpit sound levels. The package includes an insulated engine cover and additional firewall soundproofing. Transitions to HCCI mode from conventional spark ignition were abrupt and gave a noticeable shudder. GM says such transitions will be imperceptible in production vehicles, similar to the deactivation performance of the company’s production Active Fuel Management system. AFM in GM V8s runs the engines on four cylinders under low loads to save fuel.

TwinForce squeezes V8 performance from a V6

Direct fuel injection and turbocharging have long been used in diesel engines for power and fuel economy.

Direct fuel injection and turbocharging have long been used in diesel engines for power and fuel economy. Ford Motor Co., Dearborn, Mich. (ford.com), is now taking that concept, calling it TwinForce, and using it in various consumer cars and trucks. Vehicles with a TwinForce Duratec 35 V6 engine should have 415 hp and 400 lb-ft of torque, numbers usually associated with a 6-liter V8. The TwinForce V6 would also get 15% better fuel economy than a similarly powered V8, according to Ford.

Compared to conventional port-fuel injection, TwinForce direct injection more precisely controls how much and when gas is sent into the cylinders. This means combustion is more efficient. Meanwhile, twin turbochargers deliver more air to the cylinders and boost power. It also reduces emissions and the engine can burn E-85 ethanol. The Ford says its new technology will be on future Ford and Lincoln vehicles.

Fundamentals of ultraprecision machining

 

Ultraprecision machining (UPM) comes from the optics industry so not many designers are familiar with the process. However, the technology has the potential to revolutionize the way manufacturers, in general, finish parts or make fine-featured patterns.

First, recall that “high precision” in traditional machining generally refers to tolerances in the single-digit micron range. In inch units, machinists talk about holding “tenths” (ten-thousandths of an inch, or 0.0001in.). And the best conventional machining and grinding machines typically get Ra values no better than 0.1μm. In contrast, ultraprecision machining provides accuracy an order of magnitude better by holding submicron tolerances. UPM also obtains surface finish Ra values better than 0.5nm. When using diamond tooling on nonferrous materials, UPM produces yet more-impressively smooth finishes. Ametek Precitech, in Keene, N.H., manufactures UPM equipment and has provided the benchmark for this technology. Precitech’s machine layout resembles that of standard equipment, but the details make all the difference. Programming input resolution, the precision level of the machine inputs, is 0.01nm for linear and 0.026arc-sec for angular position. Work piece spindle speeds hit 18,000rpm and milling spindles rotate at 15,000 or 50,000rpm. Work piece positional accuracies of 1micron linear and ±2arc-sec are standard and — because these errors are repeatable — software compensation can be used to reduce them by a factor of 10.