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Caterpillar’s Variable Compression Ratio Engine for HCCI Combustion

Cat’s variable compression ration engine relies on an eccentrically-mounted crankshaft. Click to enlarge.

As part of its research and development into HCCI (Homogeneous Charge Compression Ignition) regimes for meeting coming heavy-duty engine emissions requirements, Caterpillar has developed—and filed a patent on—a variable compression ratio (VCR) engine.

A variable compression ratio engine offers the potential to increase combustion efficiency and decrease emissions under varying load and speed conditions.

Caterpillar is part of a Department of Energy-funded research project on high-efficiency, clean combustion along with ExxonMobil (fuels and combustion chemistry), Sandia National Laboratories (optical diagnostics, fuel spray and combustion, fuel effects), and IAV (closed loop control, transient controls, vehicle calibration, sensors).

The VCR mechanism. Click to enlarge.

Benefits of the variable compression ratio engine with an HCCI regime include high-load operation at low-compression ratios; elimination of the cold-start issue with a low-compression ratio; improvement of light-load combustion stability and the emissions of HC/CO; the optimization of load, emissions and fuel economy; and the enabling of engine braking.

The crankshaft in the 15-liter engine is mounted eccentrically in the cradle which sits in the cylinder block. Rotation of the cradle moves the crankshaft which alters the compression ratio.

Hydraulic pistons control the cradle. Click to enlarge.

Hydraulic pistons drive the cradle up and down. As a result, the engine can change its compression ratio from 8 to 15:1.

The VCR engine is by no means a panacea for highly-efficient and clean combustion, but, notes Kevin Duffy from Caterpillar, it is “a valuable development and exploration tool to understand tradeoffs of numerous controls and combustion parameters [with HCCI].

Caterpillar is not alone in its investigations of variable compression ratio engines.

Many of the earlier initiatives focused on smaller-displacement, light-duty engines. A VCR could enable significant engine downsizing in highly boosted engines, and was seen as a possible solution to bringing gasoline engine efficiency closer to that of diesels.

Among the research and development efforts:

  • In 2000 Saab introduced a prototype VCR engine at the Geneva Auto Show.

  • Three European car manufacturers (PSA Peugeot Citroën, Volvo and Renault) two powertrain development companies (FEV Motorentechnik and Le Moteur Moderne) and VKA at Aachen University of Technology worked on VCR engines as part of a 2002 European effort, Project VCR.

  • A French development company, MCE-5, is working on a variable compression ratio engine block.

  • An earlier DOE project involving Argonne National Laboratory, AVL Powertrain, Computer Systems Management, Envera and Ricardo developed a light-duty variable compression ratio engine prototype in 2001.



Rafael Seidl

Here's the 2003 paper by FEV detailling the compression variation mechanism:


Note that in addition to the eccentricity, a compensation mechanism (e.g. 4-bar linkage with identical crank lenghts) is required as the input shaft of the transmission does not rotate. The whole thing is nearly as complex as Saab's variable compression concept, in which the cylinder block can be rotated relative to the crankcase.

It's a lot easier to vary the effective compression ratio in a turbocharged engine, by adjusting the boost pressure. Apparently, Caterpillar concluded that was not sufficient for its applications given the extremely strict NOx limits that HDV diesel engines must meet by 2010 in the US. Reducing the geometric compression ratio allows clean HCCI combustion to be used at higher load levels, at the expense of higher fuel consumption.


This is good news for flex fuel vehicles. VCR engine could enable Ethnol (which has stronger "knocking" power) run more efficiently.


I was thinking that as well, jim, but the costs of such a system in mass production will probably be higher than the cost of a turbocharged vehicle and (if you could break the perception of smaller engines in the US) a small turbo flex fuel engine with boost dependant upon octane of fuel would probably work just as well as a VCR engine.

Rafael Seidl

Patrick -

I agree with you, even though this particular article refers to a 15-liter HDV engine.

Downsizing and turbocharging is a sensible strategy for fuel economy even if the fuel system is not flexible - especially when combined with (homogenous) GDI and cam phasers. If LDV manufacturers were prepared to reprogram their gasoline engine controls such that high power levels - and hence engine-out temperatures - could only be sustained very briefly, they could probably apply variable turbine geometry based on affordable alloys (unlike Porsche's solution). This sleight of hand would let them advertise both rated power and negligible turbo lag. Unless you're a speed demon, you never need rated power for extended periods anyhow.

Turbo lag is also reduced in stratified GDI but the added expense of NOx store or SCR aftertreatment makes that a bit of a non-starter. Dual fixed-geometry turbos are an option for larger engines, including V6s with cylinder deactivation.


It seems like hybrids have the low end torque with the electric motor, turbo lag may not be as big an issue there.
A small hybrid turbo FFV sedan sounds interesting to me.


Yes it does SJC. Unfortunately, a turbo can add a good couple thousand dollars to the price of an engine and then hybridization also tacks on several thousand.

Small price to pay in my mind but I'm not the same as the average consumer whom they want purchasing their vehicles in mass quantities.


Seems to me that micro-hybrid clean diesel could be a most cost-effective way to go. Get that great highway mileage and improve the city/gridlock mileage by not idling the engine.

Roger Pham

Thanks, Rafael, for the additional info. Indeed, now this VCR does not look as simple as it seemed.

In my humble opinion, it seems that an easiest way to achieve VCR is in an Atkinson-cycle engine with VVT (Variable intake Valve Timing) and variable valve lift and duration. For maximum effective compression, close the intake valve at BDC. For lower effective compression ratio, close the intake valve after BDC. In a turbo-charged engine, boost pressure can additionally vary as well for additional compression control, but this mechanism is slower, hence would be less suitable for HCCI application in which very rapid and precise control of compression ratio must be done to achieve HCCI at TDC without misfiring nor engine-destroying pre-ignition at before TDC. BMW and Honda both have technologies for variation of valve lift and duration in addition to valve timing variation (cam phaser), thus would be suitable for this application.


Where can I find the rest of DEER 2006 proceedings?


DEER 2006 presentations




This is an exact copy of FEV's VCR engine.



"This is an exact copy of FEV's VCR engine." -Terry

How can you be so sure? If that is the case then how could they apply for a patent for something that is already patented?
I don't think the change in crank centerline will be "corrected" with additional mechanisms as in FEV's VCR. It is not unheard of to have an offset crank, even in a gasoline engine.
From the picture it looks like as compression decreases the engine will decrease offset. The picture shows the midpoint as the engine is decreasing its compression ratio. More offset would allow the engine to have more favorable kinematics during high compression ratio HCCI combustion/expansion. (assuming the crankshaft and the small bar in the linkage rotates clockwise.)

david larson

Turbochargers are old news, WWII era news, turbodiesels are common enough. Ford made a FFV in 1996(?) that used a turbo to take advantage of higher ethanol anti-knock [read high octance] qualities. Of course, their idea was to feature FFV's as a horsepower booster for the 600-horsepower Mustang Stallion, but considering the willingness globally to spend more for power, perhaps highlighting ethanol's ability to provide more power via boost pressure is not a bad selling point.
A turbocharger also uses some of the otherwise wasted exhaust gas energy, and with little effort and expense, a leaner, "greener" machine.
We cannot legislate buyer's choice out of the picture. The soviets and chinese could, and thus far have environmental heritages that mirror their human rights.
Some subtle trickery is needed here: That "turbocharged V8" need only displace 2.5 liters. Its successor, ethanol POX reformer fuel cell can be "based on NASA technology." Petroleum has been using PR for a century (thanx, John D.) to 'divide and conquer' any effort to run anything but gasoline-fueled IC engines. time to wise up, stop wasting time, and say "no" to oil today.

Mike Schuch

To me it seems like there is a much better way to achieve variable compression than moving the entire crank shaft or cylinder head. I really like the concept of the pivotal piston engine shown on the concept engine section on this website. http://bioage.typepad.com/photos/uncategorized/pivotal_1.png
If you were to vary the point at which the connecting rod attaches to the pivotal piston you would be changing the stroke of the piston and the compression. It seems to me like that would be infinitely easier than trying to move the whole crankshaft or the whole cylinder head.


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The idea of varying the stroke by moving the crankshaft is not new. See:

The principle of the Rigg variable-speed Engine.

Three cylinders were pinned to a centre C, with their piston rods connecting to the periphery of a large flywheel which rotated around a fixed axis at D. The centre C could be moved to vary the eccentric distance C-D, and hence the piston stroke, which was twice C-D. The engine could be stopped by moving C in line with D, when the stroke was zero, or reversed by moving C to the other side of D.

Prof Sanjay Harip

Dear sir,

i am doing research on VCR engine at NIT. Sutrtkal, India. By r opic on this technology is very informative. I request you to send me the latest informtion about VCr engine in Animation/Video or written format to following addresss
prof Sanjay Harip
Department of Mechanical Engineering,
Government Polytechnic, Ahmednagar
Burudgaon road, Near Anand Dham,
INDIA- 91414001

wiating for possitive reply.

Thanking you
Prof Harip


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Pao Chi Pien

For a four-stroke engine, the expansion ratio and geometric compression ratio are the same and equal to the ratio between cylinder total volume and cylinder clearance volume. Compression ratio on the other hand is the ratio between the air density in cylinder clearance volume and the density of the ambient air. Since the air density in the clearance volume is controlled by a throttle valve, a four-stroke engine already has a variable compression ratio. The variable compression ratio described in the literature is actually variable expansion ratio rather than variable compression ratio.Pao Chi Pien

Account Deleted

My patents presents some stroke variation mechanisms realized by modification of the basic configuration of the mechanism, keeping constant the crank radius with the change of TDC position and displacement during operation, and with the position adjustment of BDC (and the volume of the chamber in relation to the adjusted displacement to maintain the prescribed compression ratio and the optimum conditions for combustion.
My solutions could be realized by classical technologies, with the variation of the compression ratio and the displacement (0-100%), generating high power and low consumption !
An industrial version of an engine with variable displacement mechanism (pending patent – 9 mechanisms) is under development, with perfect dynamic balance realized by classical crankshaft, rods, rotational or translational joints.

Other projects

If you are interested in any of the projects, please contact me in order to discuss details.


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