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GM Continues to Develop HCCI Platform, Demonstrates Gasoline HCCI at Idle

Speed load visitation map for a typical SI engine vehicle during the FTP driving cycle. A plausible steady-state HCCI operating zone is indicated by the region outlined in red. Point 1 represents the idle point. GM has extended HCCI operation to include idling. Diagram from Chang (2006).Click to enlarge.

GM has begun public-road demonstrations of a Saturn Aura concept vehicle equipped with a modified 2.2L Ecotec four-cylinder gasoline engine that delivers mixed mode operation: homogeneous charge compression ignition (HCCI) at lower loads, reverting to spark-ignition at higher loads and speeds. HCCI can provide up to a 15% fuel savings, according to GM.

GM first unveiled driveable HCCI concept vehicles on its test track in August 2007 (earlier post); since then, the company has added the capability of operating in HCCI mode during idle in addition to highway driving. An extended range for HCCI operation is intended as further refinements to the control system and engine hardware are made.

An HCCI engine ignites a mixture of fuel and air by compressing it in the cylinder. Unlike a spark ignition gas engine or diesel engine, HCCI produces a low-temperature, flameless release of energy throughout the entire combustion chamber. All of the fuel in the chamber is burned simultaneously.

The basic idea is to employ a premixed air-fuel mixture that is sufficiently lean or dilute to keep flame temperatures below about 1900K to help keep NOx and particulate production low. Consequently, the HCCI engine with lean burn characteristics is a very good candidate for future clean and economical passenger vehicle applications.

In spite of these great benefits, it has been very difficult to apply HCCI technology to real production engines. There are major challenges that must be overcome to make the HCCI engine practical. First, the ignition timing and combustion phasing in the HCCI engine cannot be directly controlled because there is no direct trigger, such as spark timing in SI engines or injection timing in CI engines; second, it has low power density because of its lean combustion nature; and finally, the HCCI engine has limited operating range due to knock-like rapid combustion under some conditions and misfire under others.

—Chang et. al. (2006)
Left: Traditional combustion event. Fuel enters via the intake port and a spark plug ignites the air and fuel mixture. The burn of the mixture projects from the spark and the flame progresses throughout the combustion chamber. Right: HCCI combustion event. Fuel enters via an injector in the combustion chamber. The air and fuel mixture ignites through heat caused by compression and without a spark. The lower temperature burn of the mixture is simultaneous and even throughout the combustion chamber. Click to enlarge.

To control the HCCI combustion process, the mixture composition and temperature must be changed in a complex and timely manner to achieve comparable performance of spark-ignition engines in the wide range of operating conditions. That includes extreme temperatures—both hot and cold—as well as the thin-air effect of high-altitude driving.

Heat is a necessary enabler for the HCCI process, so a traditional spark ignition is used when the engine is started cold to generate heat within the cylinders and quickly heat up the exhaust catalyst and enable HCCI operation. During HCCI mode, the mixture’s dilution is comparatively lean. This reduces the throttle losses of a conventional spark-ignited engine at low loads, helping a gasoline HCCI engine approach the efficiency of a conventional diesel, but requiring only a conventional automotive exhaust after-treatment.

Achieving HCCI operation at idle is a challenge because the relatively low-temperature and light engine load characteristics generally inhibit the proper thermodynamic conditions for successful, controllable auto-ignition. Also, heat is needed at start-up and idle to light-off the catalytic converter.

GM’s engineers overcame these challenges with advanced control of the direct injection system and an HCCI-specific cylinder pressure sensor system. After spark ignition is used to start the engine, the engine’s control system manipulates the combustion process via input from the cylinder pressure sensors so that auto-ignition can occur during idle.

Fuel consumption with a spark ignition engine is relatively high when idling, so this new development in our HCCI process helps to enhance fuel efficiency.

—Dr. Matthias Alt, HCCI program manager, GM Powertrain

Gasoline HCCI, along with other enabling advanced technologies, approaches the engine efficiency benefit of a conventional diesel, but without the need for expensive lean NOx after-treatment systems. Its efficiency comes from reduced pumping losses, burning fuel faster at lower temperatures and reducing the heat energy lost during the combustion process.

HCCI, direct injection, variable valve timing and lift, and Active Fuel Management all help improve the fuel economy and performance of our internal combustion engines. I am confident that HCCI will have a place within our portfolio of future fuel-saving technologies.

—Tom Stephens, executive vice president, GM Global Powertrain and Global Quality

The emerging HCCI technology offers several paths for implementation in a production vehicle. GM’s strategy combines the efficiency enhancements of HCCI and the power-on-demand attributes of spark ignition. This combination delivers enhanced fuel savings over a comparable, non-HCCI engine, but with the performance consumers have come to expect during higher engine load situations, such as passing or entering a freeway.

The Saturn Aura HCCI concept vehicle.

The Saturn Aura concept vehicle operates in HCCI mode up to about 55 mph (88 km/h) and switches to spark-ignition for higher-speed, higher-load conditions. It also engages spark ignition mode for passing at lower speeds and other higher-load demands.

The modified Ecotec engine produces 180 horsepower (134 kW) and 170 lb-ft of torque (230 Nm). It features a central direct-injection system, with variable valve lift on both the intake and exhaust sides, dual electric camshaft phasers and individual cylinder pressure transducers to control the combustion as well as deliver a smooth transition between combustion modes.

An advanced controller, using cylinder pressure sensors and GM-developed control algorithms, manages the HCCI combustion process, as well as the transition between HCCI combustion and conventional spark-ignition combustion.


  • Kyoungjoon Chang, Aristotelis Babajimopoulos, George A. Lavoie, Zoran S. Filipi and Dennis N. Assanis (2006) Analysis of Load and Speed Transitions in an HCCI Engine Using 1-D Cycle Simulation and Thermal Networks (SAE 2006-01-1087)



Umm, I'll take that engine without the HCCI! 2.2L 180hp & 170ft-lbs of torque - with all the goodies you can stuff into it for a possible increase in fuel efficiency? (dual cam phase control, direct injection, and variable valve lift).

Now, you do all this work to make an engine more efficient at idle...but you also add equipment to make an engine have a "start-stop system" to prevent it from idling? Which is more cost effective? Both require the engine to be warm before they are used (at least I would guess a start-stop system would require an engine to be warm before it is used).


You can leave HCCI off my order as well. A 1.4l turbo with hybrid and turbo alternator would be fine with me. It would go like a rocket and sip fuel. This may be what people are looking for, an exciting car that does not break the bank.

Dan A

I've heard that HCCI engines are idealy suited to be the gasoline engine on a hybrid because their operating range works ideally in a narrow RPM range, but is highly efficent, so the high low-RPM torque and alternating the total power, but the HCCI engine would produce the base power. It seems that what GM's doing is allowing a pure HCCI (non-hybrid) powertrain, but I don't know if this is the ideal way to go. HCCI would fit the E-flex or two-mode hybrid systems perfectly.


Seems like the only benefit is at low RPM, but the penalty for that is low HP/weight. Low RPM is precisely where electric works better.


wouldnt this be the perfect engine for the volt


Well, it also only really works in "light load". I would imagine, an efficient generator would be running at high load/peak torque for best efficiency otherwise if you run a generator at low load at low rpm then you have to have an over sized engine to get adequate power generation.


Yes, specific power is the problem for a hybrid. I don't think you want a heavier than normal genset in your serial hybrid. Stationary generators might be a better application, where you're not so concerned with power/mass ratio.

On the other hand, if you can get your ICE into HCCI mode for cruising at 65mph (a little higher than shown here), while keeping spark ignition the rest of the load cycle, you could have a very economical highway cruiser.

And I agree, HCCI at idle seems beside the point--idle-stop should become standard on all cars.

Bill W

Lets keep in mind this is installed in a car driving down the road, not a lab machine. The fact they have it on the road is fantastic. Hopefully soon to a GM car near you.

I could find allot of ways home that do not require full speed; what a challenge it would be to see how long you could keep the HCCI cycle running!! Similar to the lean burn systems that Honda sold a few years back. Every little bit counts.

Dan A

At least for a serial hybrid, and also to a large extent parallel hybrids, the only main concern for the engine is that it provides enough average power efficently. It doesn't need to provide on demand power, because the electric motors take care of most of that, and peak engine power is mostly needed during acceleration, which the electric motor is best used to handle (electric motors have the maximum torque at technically 0 RPM).


I think this would be a fantastic engine for the volt. It would probably eliminate the catalytic converter or drastically reduce it's size. The controls could probably be dumbed down because it would only be used as a generator under constant load.

factory rat

Come on guys - the point here is that HCCI is easy in the upper ranges; doing it at idle implies that lots of control problems have been solved by GM. Can the engineers weigh in? Are we not talking about 8% or so across the board on gasoline ICE with HCCI?


The point appears to be that GM can get 15% in any region up to a certain RPM/load with relatively stock 4-cylinder hardware.  I don't see any advantage for V6/V8, because cylinder deactivation will achieve most of the same results.  There's no advantage for the Volt because the sustainer would have to be bigger to compensate for lower power/weight in HCCI mode; the Volt would be best off with an Atkinson cycle, and perhaps a pressure-wave supercharger to recycle exhaust pulse energy and shrink the engine further.

15% less fuel consumption during all rush-hour traffic conditions except acceleration doesn't hold a candle to the Prius, but it's not bad; it would take you from 25 MPG to over 29 MPG.


Regarding "Fuel consumption with a spark ignition engine is relatively high when idling, so this new development in our HCCI process helps to enhance fuel efficiency."

Comparing an SI vs Turbo Diesel fuel consumption:

1. At idle ~ 3:1
2. in between ~ (>3 but <1.3):1
3. Maximum power ~ 1.3:1

BTW 1. and 2. are the reasons diesels more often than not get much better MPG than the fuel price difference.

One would hope that the 3:1 could be improved with HCCI closer to 1.3:1, but that would probably require higher comnpression ratio like a diesel.

So the comment "HCCI can provide up to a 15% fuel savings, according to GM" leads me to think the improvement is still nowhere near the diesel since HCCI is only at low load where diesel has the biggest advantage.

I I misunderstand the % improvement. Is it over a standard driving cycle, city, highway?

On the plus side this looks like potentially cheap to produce, aside the direct injection.


The improvement of 15% doesn't look very exciting, you can get this with a direct injection, it seems to me that HCCI could return a better improvement of efficiency.

Brian P

Homogeneous direct-injection has not given 15% improvement in practice.

The 15% can be achieved with direct-injection and lean-burn, but then NOx is a problem.

The HCCI gives you the 15% improvement AND the compliance with NOx.

I don't see what all the bellyachers are bellyaching about. Hopefully GM puts this into production.

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