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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.


I do not think it is a matter of bellies nor aching, it is a rational discussion of the relative merits on the topic. Just stating that it passes your test and you command that it be produced is not going to have much of an impact.

Jay Tee

HCCI is supposed to offer as much as 40 % in fuel savings. Only GM could come away with 15 %


No its a typical EV, hybrid biased rant from all the clueless. HCCI is the way of the future as all engines, especially diesels will use it to pass emissions WITHOUT aftertreatment. Meaning NO DPF and the fuel economy loss from them. Its atleast 15% free fuel economy gain and you idiots whine because its not good enough? When will anything be good enough for you fantasy land dwellers? Welcome to reality.
This is a big deal that this is going into production.


"Only GM could come away with 15 %"

No, China will get an amazing 5.5% after they steal the technology.


Why go to all that trouble to get improved mileage at idle and reduced power when 15 to 40% improvement can be achieved with a supplemental, on-demand hydrogen generation system using electrolysis.

Supplemental hydrogen has been researched and proven to be simple and economical by a couple of NASA engineers that did the research for the Society of Automotive Engineers.

There are presently over two dozen companies selling supplemental HHO systems for as low as $250 for autos and pickups up to over $2000 for systems used on 18-wheelers.

All that is required is a small electrolyzer to feed HHO gas (Brown's gas) to the intake on the engine. A wide band O2 sensor is needed to adjust the mixture for a 20:1 A/F ratio.


After reading many of your post, I have some comments.

HCCI may in fact be the Holy Grail. Much lower emissions without after-treatment not to mention reduction in fuel consumption in the 15 to 40% range, and this is with current drive-trains. This has been discussed widely in magazines such as SAE's monthly magazine.

I had a friend in this arena before being downsized, and while these are off the record comments, here is the gist of a conversation we had on the topic. For one this would not work in a General Aviation Engine. You would have to make is so big to run in the sweet spot it is not worth it. When I asked him is this sort of controlled pinging? His answer was yes basically. The problem what I understand (and I only stayed at a Holiday Inn Express Last night) is the transition out of the mode.

Look, GM has said they are 5 to 7 years away to production intent with their current HCCI efforts.

Where I think the sweet spots will be is with Series Hybrids such as the Volt. Rather than running a variable "Air-Pump" like we have for 100 yrs, the engine is optimized for a given rpm, and it's lowest Brake Specific Fuel Consumption. (BSFC). This is what the critics of the Volt and GM do not get, if they take an intro course in engines or do some heavy reading on their own maybe they will figure this out.

Why not make male a car like the Volt run in HCCI mode at that design point and make it an ever bigger win-win, This is not a new concept, it has been bandied about on the green chat rooms for a while. My guess is it may have to be a bit bigger, unless you can work miracles moving the HP and Torque curves down to this lower RPM and still turn your generator for the desired output.

Better yet let us shoot for an Omnivorous engine that is HCCI. That way you can burn anything. Look I am not fond of Diesels, I think in the long run HCCI may make diesels boat anchors. It is my wild guess it will be easier to get a Piston Engine that is HCCI to run on anything vs. a Diesel to run on anything.

And for all those that do not think this is good enough, find some venture capital and start tinkering in your garage. Or, take an engineer for the greater Metro Detroit area working on these projects out for a cup of coffee or a beer and see how easy it is after he let or she lets you walk in their shoes for a while.....

I you find any of these comments technically incorrect as someone who may be working in this field, I would like to hear your corrections and I look forward to the greater understanding it may bring.



Remember how you started off with, "this would not work in a General Aviation Engine. You would have to make is[sic] so big to run in the sweet spot it is not worth it."

Don't forget to apply that to a serial hybrid drivetrain as well.

A 2.2L 4-cylinder engine in HCCI mode would probably work for generation needs of a "Volt"...but why go with such an oversized engine to get 15% better fuel economy than a typical 2.2L as a generator [for a serial hybrid] when you really only need a 0.6L or 1.0L 2 or 3 cylinder motor to provide all of your generation needs? I bet you can do much better than a 15% improvement by cutting the displacement in half and running the motor at higher load (to maintain the same output as the 2.2L HCCI mode output).

HCCI - Good for a standard ICE - powered vehicle, but I don't know about as a generator for a serial hybrid. I would imagine a light duty truck with a V-6 or V-8 is often at very light load while cruising down the highway and could go into HCCI mode for any long trip.

Andrey Levin

This is great achievement.

Gasoline engine has max thermal efficiency at full throttle, however for ground vehicles it never runs at full throttle for prolonged time – exhaust is too hot, too high noise, vibration, and engine wear.

Stratified charge lean burn gasoline engine could happily run near full throttle with almost diesel-like efficiency (standard technology on boat outboard engines), but 3-way catalytic converter does not work with excess oxygen in exhaust.

There are two way to solve this problem: expensive and capricious NOx adsorber catalyst, or much simpler way – HCCI.

Rafael Seidl

HCCI is one of many different approaches being pursued to improve the fuel efficiency of gasoline engines. Combinations of these approaches often yield even greater improvements, but unfortunately system cost and complexity increases faster than the fuel economy gains.

For example, once the stored grid electricity runs out, the GM Volt will operate as a series hybrid with a large energy buffer. In that configuration, you'd want the engine to run near its operating point of best specific fuel consumption and use the battery to mask the peaks and valleys of instantaneous demand. However, you only want to maintain the average charge level, so if it exceeds a certain threshold you should just switch off the ICE. Efficient ICE operation means running at moderate RPM and high load to minimize internal friction losses - this is outside the portion of the map suitable for HCCI.

The story is different for concepts that do not involve a big battery. Passenger cars feature engines that are much more powerful than they need to be for 95% of the duty cycle. The obvious approach of using not one large but two small engines is complex and expensive. Instead, many engineers are trying to squeeze more rated power out of just one relatively small engine via GDI and turbocharging plus advanced transmissions that keep the ICE running at - drumroll please - moderate RPM and relatively high load.

Gasoline HCCI delivers its greatest fuel economy improvement for large displacement passenger car engines that spend a large fraction of their anticipated duty cycle in inefficient low load. Especially in the US market, V8s and big V6s still command higher higher premiums than advanced I4s, let alone balanced I3s.

Diesel HCCI is a different animal, its primary purpose is to reduce NOx emissions. Unfortunately, it does not eliminate the need for a DPF and NOx aftertreatment for the regular CI operation that becomes necessary at high load - which means it will only be applied if there is no other way to meet tightening emissions regulations.


Good output at lower RPMs seems like a good idea for series hybrids at first glance. It would be nicer to have an engine running at 3000 rpm putting out 60 hp than one running at 5000 rpm putting out 90. I know those numbers probably do not match the graphs, but you get my point.

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