|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|
|Phasing of event||Unrestricted|
|Max. velocity||5 ms|
|Max. engine speed||7,400 rpm (gasoline/HSDI)|
2,400 rpm (heavy-duty diesel)
|Residual cylinder pressure||20 bar|
(70 bar for exhaust braking)
|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.
“Active valve train for homogeneous charge compression ignition”; N. Milovanovic, J.W.G. Turner, S.A. Kenchington, G. Pitcher, D.W. Blundell; International Journal of Engine Research, Volume 6, Number 4, July 2005, pp. 377-397(21