Mazda L3-VDT engine

Introduction

The Mazda L3-VDT was a 2.3-litre turbocharged four-cylinder petrol engine with direct fuel injection that was first introduced in the Mazda GG Mazda6 MPS in 2005. The L3-VDT engine was subsequently deployed in the Mazda BK Mazda3 MPS, BL Mazda3 MPS and Mazda CX-7.

Block

The L3-VDT engine had a precision gravity sand-cast cylinder block that was made of high-grade aluminium alloy, while cast-iron cylinder sleeves were cast directly into the block. With 87.5 mm bores and a 94.0 mm stroke, the L3-VDT engine had capacity of 2261 cc. The cylinder block was a deep skirt type and formed a ladder frame structure with the integrated main bearing cap. The water jacket of the cylinder block was a closed-deck type.

The L3-VDT engine had a cassette type balancer unit which consisted of two balancer shafts that rotated at twice the speed of the crankshaft to offset the secondary inertia force of the rotating system. The balance shaft was fitted in a two-piece housing that was fixed to the cylinder block by four bolts; it was driven by a gear attached to the crankshaft to reduce drive losses.

Crankshaft, connecting rods and pistons

The L3-VDT engine had a forged steel crankshaft with eight counterweights which operated on five bearings; to prevent movement, the crankshaft pulley had a torsional damper.

The L3-VDT engine had sinter-forged connecting rods with fraction-split ends. Compared to the naturally aspirated L3 engine, the shape of the aluminium alloy pistons for the L3-VDT was adapted for the direct injection system with ‘dished’ tops to improve mixture formation. The piston skirt was also redesigned due to the addition of oil jets to lubricate and cool the pistons.

Unlike the L3 engine, the pistons, piston pins and connecting rods for the L3-VDT engine were not ‘shrinkage fitted’ and could therefore be replaced separately. Maximum engine speed for the L3-VDT engine was 6500 rpm.

Cylinder head and valvetrain

The cylinder head for the L3-VDT engine was made from aluminium-silicon alloy which was thermally treated for strength and durability after the casting process. For the L3-VDT engine, the cylinder head was mounted on a four-layer metal head gasket.

Whereas the L3 engine had a pinned link timing chain, the double overhead camshafts for the L3-VDT engine were driven by a more durable roller chain. Furthermore, the intake camshaft for the L3-VDT engine had a trochoid shaped cam which drove the high-pressure pump for the direct injection system. Similarly, however, the timing chain was automatically tensioned using oil pressure and the spring force in the chain tensioner.

The four valves per cylinder – which had an included valve angle of 39 degrees – were actuated by mechanical bucket tappets. The tappets were precision-fitted during assembly and did not require adjustment shims; instead, valve clearance was adjusted by the different thickness of the bucket tappets. To improve abrasion ressitance, the intake and exhaust valves were subject to the ‘Tufftride’ (ferritic nitrocarburising) process. Intake valve lift was 9.1 mm, while exhaust valve lift was 7.7 mm.

Borg Warner K04 Turbocharger and intercooler

The L3-VDT engine had a a single-scroll, fixed geometry turbine (FGT) Borg Warner KKK K0422-882 turbocharger which provided peak boost pressure of 1.076 bar (15.6 psi); according to Mazda, boost was ‘available at engine speeds as low as 2500 rpm’. The turbocharger had:

• A waste gate valve which allowed exhaust pressure to bypass the turbine and dump directly into the exhaust; and,
• An air bypass valve which protected the intake manifold air ducts, passages and sealing gaskets from damage due to over-boost. Over-boost could occur when accelerating the vehicle and closing the throttle suddenly.

The L3-VDT engine had an air-to-air intercooler that was mounted on top of the engine. Unlike the L3-VE engine, the intake manifold and valve cover for the L3-VDT engine were made from aluminium due to its better heat and pressure characteristics.

L3-VDT: Sequential Valve Timing (S-VT)

For the L3-VDT engine, Mazda’s ‘Sequential Valve Timing’ (S-VT) provided variable intake valve timing. The variable valve timing actuator consisted of a housing case that was integrated to the camshaft sprocket, a cover, a camshaft-integrated rotor and a stopper pin that locked the rotor and case when the engine stopped. The variable valve timing actuator contained two hydraulic chambers – a valve timing advance chamber and a valve timing retard chamber – which were located between the integrated housing of the camshaft sprocket and the camshaft integrated rotor. An oil pump supplied engine oil to each chamber and this pressure – controlled by the oil control valve (OCV) – varied the position of the camshaft relative to the sprocket.

Strategies for the S-VT may be summarised as follows:

• When idling or under light loads, valve overlap would be reduced so that less exhaust gas would be returned to the intake port – this stabilised idle speed and improved fuel economy;
• At medium loads, valve overlap would be increased to increase the exhaust gas recirculation ratio inside the cylinder – this reduced engine friction loss (pumping loss), lowered the combustion tempeature and reduced the nitrous oxides in exhaust gases;
• Under heavy loads and low-to-middle engine speeds, the intake valve would close early to enhance torque;
• Under heavy loads at high engine speeds, intake valve closure would be delayed to enhance power; and,
• At low temperatures, overlap would be minimised to prevent combusted gas from returning to the intake port and to reduce the additional fuel injection amount – this improved fuel economy and stabilised fast idle speed.

From the table above, valve overlap for the L3-VDT engine range from 5 to 35 degrees. Furthermore, intake duration was 245 degrees and exhaust duration was 227 degrees.

Direct Injection Spark Ignition (DISI)

The L3-VDT engine had a direct injection system which Mazda referred to as ‘Direct Injection Spark Ignition’ (DISI). The DISI system included:

• An in-tank electric fuel pump which provided pressure of 4.1 to 4.9 bar (60-71 psi);
• A mechanical fuel pump driven by the camshaft which provided pressure of 28.3 bar to 130 bar (410-1885 psi);
• A fuel rail which supplied fuel to the injectors; and,
• A pressure relief valve inside the low pressure fuel return line which allowed pressure over 1885 psi to return to the low pressure side of the high pressure fuel pump.

Direct injection produced a homogeneous fuel:air mixture around the spark plugs, effectively retarding misfire. The ignition timing could then be retarded to help raise exhaust-gas tempeatures for fast warm-up of the ctalytic converter.

The L3-VDT had a compression ratio of 9.5:1.