The Nissan SR-Series Engines

The SR-series of Nissan engines (perhaps the best-known is the potent and easily upgradeable SR20DET turbo 2-litre) was designed by Nissan as a replacement for the earlier CA series of engines. Here’s how the engineers went about their task.

The Aims

The development aims stated by the engineers for the engine was:

• Excellent power output and ample torque in the middle to high engine rpm ranges
• A good balance between high output and fuel economy
• A “clear, linear sound” up to the redline

Oddly enough, except for a throwaway line (“good adaptability to future emission control regulation”), objectives relating to emissions were missing from the original discussion – perhaps one reason why after a production lifetime of about 13 years, emissions performance killed the design.

Overview

Unlike many contemporary engine designs which were being constantly upsized (for example, from 2 to 2.2 or 2.4 litres), the engineers stuck with the ‘traditional’ 2-litre swept volume, gained from a bore and stroke each of 86mm. To keep engine mass low, an aluminium block was used. This design used a closed upper deck and a skirt which extended well below the crankshaft centreline. The head used compact, pentroof combustion chambers (cross-flow, of course), with the sparkplug located in the centre of the combustion chamber.

The Head

The straight intake ports were designed with what Nissan terms an ‘aerodynamic port shape’, that is, a port that decreases in cross-sectional area as the runner/intake port gets closer to the intake valve. The ports were also positioned high in the head, so reducing the angle which the air had to negotiate before entering the combustion chamber. In addition to improving flow, the greater length of the port served to improve torque through giving a longer tuned length to the intake port/intake runner combination. In fact, a combined intake port/runner length of 450mm was used in the SR20DE engine. Compared with a low port design, the high port design improved measured torque by 4 – 7 ft-lb at engine speeds of over 4400 rpm.

A narrow valve angle of 29 degrees was selected – this can be compared with the 45-degree angle of the CA engines.  Over the CA18DE, the SR20DE engine provided a 2.7 per cent improvement in its brake specific fuel consumption at the same torque output (one which corresponded to 60 km/h road speed, presumably in top gear).

The high ports required that the camshafts worked the valves through Y-shaped rocker arms pivoted between the intake ports. The rockers featured reduced contact area with the cam followers and also reduced inertial weight over other designs, so reducing valvetrain friction.  The cams were chain driven.

The exhaust side of the head was designed last. To reduce exhaust interference effects, the exhaust manifold used a design where cylinders 1 and 4, and 2 and 3 were each combined into a dual manifold – standard practice on most four cylinder engines. The dual part of the exhaust extended as far rearwards as the rear part of the sump.

Nissan engineers very carefully considered the structural options open to them before settling on a DOHC, narrow valve angle head with the valves operated through outer pivoted Y-shaped rockers. Other options considered included:

• a similar design to the final iteration, except for the use of one rocker per valve and the use of inner pivots for the rocker arms (the negatives were greater mass – and so friction - in the valvetrain, and a very wide head)
• direct-operated valves with a wider valve angle (negatives: wide head, couldn’t use roller rockers, built-in hydraulic lifters would add to masses, valve lift restriction because of bucket size)
• direct operated valves with a narrow valve angle (negatives: couldn’t use roller rockers, built-in hydraulic lifters would add to masses, valve lift restriction because of bucket size)

The Block

The SR20 engine was developed around the time of the 4.5-litre VH45 V8 engine used in the Infiniti luxury car. Both engines used Low Pressure Die Cast alloy blocks and drew on the experience gained by Nissan with their first alloy block engine, the 1-litre MA series. The technology jump was a large one, although the weight saving of the SR20 block over the CA18/20 iron block was only 9kg. However, as Nissan engineers of the time suggested, “attention was devoted not merely to reduce weight, but to assure functional reliability and to improve NVH [noise, vibration, harshness] characteristics”.

The block alloy was heat-treated JIS AC2A, a material already used by Nissan in cylinder heads. A closed-deck design was adopted for these reasons:

• cylinder head gasket sealability
• improved NVH characteristics
• reduced permanent bore distortion

To test the reduction in NVH of a closed deck, Nissan engineers had a CA20 block cast in aluminium (the CA being an open deck design) and then tested it back-to-back with a prototype closed-deck SR20. In the closed-deck design the greater stiffness of the water jacket wall reduced radiant noise by 2-3dB over the whole engine rev range.

Other Nissan testing showed that cylinder bore distortion of the cast-in iron liners could reach 0.05mm in an open deck design, causing piston slap noise and oil consumption problems. This permanent distortion was reduced to 0.04mm in the closed deck version.

A deep skirt block design was adopted to improve powertrain rigidity – but to give reduced NVH, rather than improved engine strength. To prove this, the engineers tested a modified VG30 3-litre V6 Nisan engine in deep skirt and half skirt block configurations. In deep skirt guise the engine showed a clear increase in frequencies, indicative of a stiffer powertrain. In addition, the connection to the gearbox was improved by the use of a two-piece oil pan, with the upper half an aluminium casting. This was needed particularly because of the Pulsar GTiR application, where the engineers were concerned with the mass of the four-wheel drive transfer case and differential being mounted on the side of the engine and transmission.

A cast-in-place dry iron liner was used within the aluminium block. However, unlike many other engines of this construction, the iron liners were ‘buried’ beneath the surface of the block. This was done for two reasons:

• To improve cylinder head gasket sealability
• To improve the life of machining tools (the resulting gain was 5 -10 times)

The Sound

Nissan engineers said that they placed great importance on both reducing generated engine noise and also creating “improved sound tone and quality”. To this end, they concentrated on the rigidity of the engine (and of the engine/trans combination, which used 10 connecting bolts) by using a main bearing girdle, the two-piece sump and the closed-deck block. (It’s interesting to consider that it’s quite likely that much of the engine’s legendary durability came about through technologies aimed at improving the quality of the sounds made by the engine....)

Reliability and Maintenance

Nissan bucked the trend of going to cam belts by retaining chain drive for the twin cams. The single roller chains used a tensioner applying force through the use both of oil pressure and springs.

Unusually for the time, platinum tipped plugs were used which were said to not require adjustment or replacement for 100,000km. Also aiding reliability were cylinder head bolts which were tensioned such that they stretched an appropriate amount and a “large size distributor [that allowed] the necessary secondary voltage for up to 7500 rpm” to be available.

The 16-bit ECU controlled bottom-feed injectors with a high heat resistance and a hot film type airflow meter was fitted. In addition, the engine management system included control of the air/fuel ratio (with learning control from an oxy sensor), knock control and self-learning idle speed control.

Conclusion

Nissan spent about 3 years developing the SR series of engines – including the SR20DE, SR20DET, SR20DI, SR18DE and SR18DI.

Said the engineers: “We believe that the goal of this development, namely a good balance between high performance and fuel economy combined with a pleasant engine sound, has been attained by the design technique of the 4 valve DOHC engine which Nissan has been cultivating over many years, and by the solid reliability built into this car by Nissan’s production technology.”

The Specs Type: Water cooled, 4 cycle in-line 4 cylinder Combustion Chamber: Cross flow, pentroof type Valve mechanism: DOHC, 4 valves per cylinder, chain drive Displacement: 1998cc Bore x Stroke: 86.0 x 86.0mm Bore Pitch: 97.0mm Block Height: 211.3mm Compression ratio: 9.5:1 Crankshaft journal diameter: 55.0mm Crankpin diameter: 48.0mm Con rod length: 136.3mm Valve diameters: In: 34.0mm, Exh: 30.0mm Dimensions: 685 x 610 x 615mm Maximum power: 140hp at 6400 rpm (SAE net) Maximum torque: 132 ft-lb at 4800 rpm (SAE net)

SAE paper 901714 – Development of the Four Cylinder SR Engine SAE paper 910431 – Nissan’s New V8 and L4 Aluminium Cylinder Block – Design and Production Note: Unfortunately Nissan engineers did not release a paper specifically  on the development of the SR20DET turbo version of the engine. Perhaps they thought that with that particular version, they were too far ahead of the field to give away company secrets...