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Lotus Engineering Developing Advanced Combustion System with Air-Hybrid Capability

AVT sub-module for inlet or exhaust valve. Click to enlarge.

Lotus Engineering, in partnership with Eaton, is working on enabling advanced combustion control based on an electro-hydraulic valve system with full flexible control over valve timing, lift and velocity.

The fully variable valve timing system, known as Active Valve Train (AVT), will allow application of advanced engine control strategies such as HCCI (Homogeneous Charge Compression Ignition) systems, mixed-mode implementation of HCCI and convention spark ignition (SI) or compression ignition (CI), fast start, variable firing order, differential cylinder loading—and ultimately, air hybridization, according to Lotus engineers.

AVT offers the ability to run different valve profiles (trapezoidal or triangular) and to open and close valves more than once per engine cycle. The valve system could support a pneumatic hybrid application in which air is pumped to a receiver during vehicle braking. The engine would then function as an air motor for launch, and have access to enhanced turbocharging capability.

Background on AVT. Lotus Engineering has a long history of developing fast-acting electrohydraulic systems, including applications in winning Formula 1 racing cars. When the company set out to develop a fully variable valve train system, it first had to decide between electromagnetic and electrohydraulic actuation.

Based on a number of concerns about electromagnetic systems (method of actuation, control of force, lack of real-time positional feedback control), Lotus decided to go electrohydraulic.

Production AVT targets
Lift 0–15 mm, cont. variable
Valve opening/
closing timing
Phasing of event Unrestricted
Max. velocity 5 ms
Valve operation Individual
Max. engine speed 7,400 rpm (gasoline/HSDI)
2,400 rpm (heavy-duty diesel)
Residual cylinder pressure 20 bar
(70 bar for exhaust braking)
Lift repeatability 1%
Timing repeatability 1º Crank Angle

A research-grade version of the electrohydraulic AVT system has been in development now for more than 10 years, and has been applied to gasoline, diesel and natural gas engines. For the production AVT, Lotus teamed with Eaton to develop a simplified (and less expensive) version of the more complex research AVT.

A switching valve (on/off valve) directs flow either to or from actuator valves, one per engine poppet valve, depending upon whether the poppet valve is to be commanded to move open or closed.

As an example of one sequence of operation:

  • Switching valve is on “pressure”, with the actuator valve closed;

  • Actuator valve then positively opens to enable opening, then closes near to peak lift;

  • Approximately half-way through the valve event, the switching valve moves to “return”;

  • Positive opening of the actuator valve allows the valve to close as a result of strain energy stored in the return spring during the opening event;

  • Near to the seat, the actuator valve closes, and control system and reduced spring load give a soft touchdown, with the facility to tailor valve overlap.

A closed loop controls the valve position, with the position sensor located inside the actuator body.

AVT and advanced combustion. While HCCI in theory will contribute to reduced emissions and enhanced fuel efficiency, implementing the regime poses a number of challenges. Among these are:

  • Enlarging HCCI operational area towards higher loads, i.e. reducing a high rate of heat release

  • Reducing excessive CO and HC emission at low loads

  • Obtaining smooth transition in mixed mode CI-HCCI engines

  • Increasing efficiency (and pressure ratio / air flow) of charging system

  • Keeping existing engine geometry unchanged

Lotus is running a series of tests on single and multi-cylinder gasoline and diesel engines using the AVT to enable HCCI, and to support fast and smooth transition in mixed-mode operation.

Some of the results of the work include:

  • Demonstration of initiation of and controlled HCCI combustion in a certain load/speed range in a gasoline HCCI-SI mixed-mode engine, using an early exhaust valve close (EVC) with a late inlet valve open (IVO) recompression method;

  • Improved mode transition compared to that performed with cam profile switching and phase systems;

  • The ability to extend the HCCI operating range toward higher loads by influencing the effective compression ration and by using a combination of recompression and re breathing strategies;

  • The potential to enable HCCI operations at low loads by using a short exhaust event;

  • The ability to influence the effective compression ratio/pressure in a diesel HCCI-CI engine by early intake valve close (EIVC) and late inlet valve close (LIVC) and therefore increase the usable HCCI operational area;

  • The potential to reduce HC and CO emissions at low loads without deterioration in fuel consumption through the use of different valve strategies, including single and double exhaust valve opening, increasing the negative valve overlap, shifting the exhaust and inlet profile, and exhaust valve lift reduction.

Lotus has shown a reduction in NOx emissions below the upcoming 2010 requirements, along with torque increase from 50% to 100% and reduction in fuel consumption of 10% to 15%.

Honda, for one, has been vocal about developing an HCCI-type engine in a hybrid application that could result in a new Civic hybrid achieving as much as 65 mpg—30% better than the new 2006 version. (Earlier post.)

Air hybridization. There is another potential hybrid benefit from an AVT-HCCI engine—the possibility of air hybridization. Lotus has begun initial testing of that type of application.

A two-liter 4-cylinder engine with AVT can charge a 30 liter air reservoir to 22 bar in 12 seconds with the engine driven at 5000 rev / min (85% achieved in 6 seconds). That compressed air would then be used for launch assist.

The AVT system also enables new turbocharging applications that could improve performance and reduce pumping loses.



Rafael Seidl

Camless valve drives, both purely electromechanical and electrohydraulic, have been in development for quite some time (e.g. at AVL in Austria & FEV in Germany).

The primary snag is the high force required to open an exhaust valve. This makes the valve actuators rather heavy and power-hungry. The only true fully variable valvetrain design in series production today is BMW's mechanical Valvetronic. It eliminates the need for a throttle but is very expensive to manufacture and does not permit deactivation of individual cylinders.


Details on the pneumatic hybrid concept are here:

In principle, this looks like a promising concept for downsized turbocharged gasoline engines. Restarting a hot engine with pressurized air can be tricky, but it could certainly assist an electric start motor. It's not clear to me why you would need a fancy camless valve drive, a few strategically placed valves (butterfly or wastegate type) in the intake and exhaust manifolds should do the trick as well. The main problem is the size of the pressure tank/accumulator and its intercooler.


Sorry, the BMW Valvetronic system is not a "true" fully variable valvetrain. Valvetronic has only been applied to the INTAKE side of the head and valvetrain.

Valeo has a fully working head with electromechanical valves. The snag (for it and all other camless electromechanical valvetrains I've heard of) is the high current levels required on pretty much MUST use 42V to supply enough power for the valvetrain.


Electrohydraulic exhaust valve actuation is routinely used on big two-stroke marine diesel engines. Hopefully it will work on smaller engines too.


It seems most HCCI projects are targeting increased efficiency using gasoline, but does ethanol lend itself well to such a design? One would think the high octane of ethanol would be a benefit. An HCCI ethanol engine would likely achieve higher MPG than a standard gasoline engine, making up for much of it's lower energy density....

Rafael Seidl

Patrick -

the variable lift portion of Valvetronic is indeed only implemented on the intake side, because its purpose is to replace the inefficient butterfly valve normally used to control the density of the fresh charge. BMW still includes a butterfly valve but it is only used in emergency situations.

There is no point in making the exhaust valve lift variable. Both intake and exhaust camshafts feature a cam phaser, though.


Angelo -

HCCI et al. are intended to sharply reduce engine-out emissions of NOx at the expense of HC and CO. For lambda=1 engines, this is valuable in the warm-up phase of the three-way catalyst (first 2 minutes or so). The initial HC peak can be captured in an adsorber and cleaned up later.

Volume ignition requires very high AGR rates and cycle-accurate valve control, which is hard/expensive. Because of the extremely rapid combustion, mechanical stresses and combustion noise limit volume ignition to low loads. For higher loads, you have to revert to traditional ignition, but making the transition stable and transparent to the driver is very difficult.

Any fuel economy gains are exclusively due to the fact that the engine can be operated at lambda>1 at low loads, reducing losses in engines that have throttles.

In theroy, high ethanol blends like E85 could be combusted using a suitable volume ignition strategy, but the range penalty due to the low energy content per gallon relative to gasoline would remain. Moreover, the high octane rating would neccessitate high compression ratios, forcing the transition line to traditional spark ignition to lower torque values.


No-one ever thougt of implementing Atkinson Cycle transition operation for low loads? Or does the compression suffers then?

Ravinder  singh

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