Mazda L5-VE engine

Introduction

The Mazda L5-VE was a 2.5-litre four-cylinder petrol engine that powered the Mazda GH Mazda6 (2008-12), the Mazda BL Mazda3 SP25 (2009-13) and the Mazda CX-7 (2009-12).

Block

The L5-VE engine had an aluminium alloy cylinder block with 4340 steel-molybdenum alloy bores for their high heat tolerance and low-friction properties. With 89.0 mm bores and a 100.0 mm stroke, the L5-VE engine had capacity of 2488 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 L5-VE 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. To reduce drive losses, the balance shafts were driven by a gear that was attached to the crankshaft.

Crankshaft, connecting rods and pistons

The L5-VE engine had a forged steel crankshaft with eight counterweights which operated on five bearings. To reduce resonances at engine speeds between 2000 and 5000 rpm, a flexible joint was used between the crankshaft and the flywheel.

The L5-VE engine had forged powder-metal connecting rods, while the aluminium alloy pistons had graphite-coated skirts to reduce friction; to reduce piston-slapping noise, the pistons were offset by 0.8 mm. The connecting rod, piston and piston pin were shrinkage fitted and could therefore not be disassembled.

The L5-VE engine had a redline of 6200 rpm.

Cylinder head and valvetrain

The cylinder head for the L5-VE engine was made from aluminium-silicon alloy which was thermally treated for strength and durability after the casting process. The double overhead camshafts were driven by a maintenance-free, link grounding type chain; the timing chain was automatically tensioned using oil pressure and the spring force in the chain tensioner. To improve abrasion resistance, the pin part of the chain was nitride-treated.

The four valves per cylinder were actuated by shimless, 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 resistance, the intake and exhaust valves were subject to the ‘Tufftride’ (ferritic nitrocarburising) process. Intake valve lift for the L5-VE engine was 9.1 mm; exhaust valve lift was 7.8 mm.

LL5-VE: Sequential Valve Timing (S-VT)

For the L5-VE 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.

Valve timing

From the table below, valve overlap for the L5-VE engine in the Mazda GH Mazda6 ranged from 0 to 30 degrees relative to the crankshaft; intake duration was 242 degrees, while exhaust duration was 227 degrees.

Variable Tumble Control System

The L5-VE engine had a friction-welded nylon intake manifold with equal length runners. Close to the cylinder head flange, the runners were fitted with tumble swirl control valves (TSCVs) which were controlled by a solenoid actuator. During light-load operations such, the valves would open to increase the velocity of the intake charge, improve mixture formation and reduce emissions. At higher engine speeds, the valves would open to enhance power output.

Variable Length Intake Manifold

The L5-VE engine had a variable length intake manifold that was referred to by Mazda as a variable induction system or VIS. At low engine speeds, the intake runners would have a longer path to increase the pulsing effect of the airflow and draw more air into the cylinder. At higher engine speeds, however, a valve would open to shorten the length of the intake runners, reduce intake resistance and allow a greater volume of air into the cylinder for top-end power.

Injection and ignition

The L5-VE engine had sequential, multi-point electronic injection and distributorless ignition system (i.e. direct ignition coils). Furthermore, the L5-VE engine had pentroof combustion chambers and a compression ratio of 9.7:1.

Exhaust

To reduce engine back-pressure and quickly heat the catalytic converter, Mazda used a fabricated stainless steel exhaust manifold with welded tubes. The exhaust manifold had a heat-insulating layer which surrounded the area where the four runners combined and a longitudinally-mounted divider plate in the exhaust pipe. According to Mazda, this design:

• Combined the heat retention benefits of a single pipe structure with the engine performance benefits of a dual-pipe structure;
• Eliminated the need for a close-coupled catalytic converter; and,
• Reduced back pressure.

For the BL Mazda3 SP25, Mazda introduced a new catalyst material structure that allowed precious metal particles smaller than 5 nanometres in diameter to be embedded in it. In contrast to previous catalysts, these ‘single nano particles’ could remain fixed in their original positions when exposed to exhaust gases to maintain their surface area and prevent them from combining with other particles. As a result, the three-way catalyst for the Mazda BL Mazda3 had 70 per cent less precious metals in it than its predecessor. This catalyst structure was also used for the L5-VE engine in the Mazda CX-7.