Saturn Grows Up

Automotive Industries, July, 1999 by Gerry Kobe

The basic architecture -- two-piece aluminum cylinder block with separate lower crankcase/bedplate, 4-bolt main bearing caps, structural die-cast oil pan, chain-driven dohc -- is designed for 150,000 miles/10 years durability with long service intervals. It also provides plenty of development headroom. Future L850 derivatives will include gasoline direct fuel injection, cylinder deactivation, variable valve timing (VVT), variable-geometry intake manifolding, and turbocharging (for Opel's 2000 Astra coupe, Saab and perhaps other brands). Though the Saturn engine has none of these features yet, it has passages cast-in for eventual secondary air injection and EGR and an oil gallery for piston cooling jets on the turbo version. The turbo engine also uses a forged steel crankshaft; the standard crank is nodular cast iron.

The program began in 1995, with a joint American-German engineering team headed by GM Powertrain Division boss Dr.-Ing. Otto Willenbockel. Lotus Engineering in England provided development support.

Initially, the Saturn engine meets California LEV emission standards, though the program is working to certify it to the more stringent ULEV. To achieve this, the engine features a combustion chamber optimized more for overall aerodynamics than maximum swirl. Ports are nearly horizontal, and the 9.9mm inlet and 10.1mm exhaust valves are angled at 21 degrees and 26 degrees, respectively, from vertical. The top piston ring land is located only 3mm from the crown, to minimize crevices that trap unburned fuel.

Fast warm up, critical for LEV catalyst effectiveness, is aided in a number of ways. SM's lost-foam casting process allows a thin (8mm) roof between the combustion chambers and coolant passages in the head. The L850 is designed for transverse mounting, and it's mounted in the Saturn with the exhaust ports facing the firewall. Though this requires extra bulkhead insulation, it shields the cast-iron exhaust manifold from cooling air and shortens the distance to the underfloor catalytic converter. The ULEV version will get even better exhaust heat retention and an engine bay pre-cat.

To minimize friction, the valves operate via roller-finger followers, pivoting on hydraulic fulcrums to eliminate lash adjustment. Twin cast-iron balance shafts are chain driven at twice crank speed. The gerotor-type oil pump is concentric with the crankshaft nose. The L850's oil filter is mounted on the front of the block, easily accessible from above and features a recyclable filter element.

The cooling system layout is unusual. The thermostat is on the inlet side, rather than outlet side, of the chain-driven water pump. GM claims this shields the thermostat from large flows of cold coolant during warm-up --the cause of large cyclical temperature variations in the block, and appreciable thermal shock.

External bracketry is absent from the 2.2L Accessories bolt to cast-in bosses on the block. Cast-in features include the timing chain case, and the continuation of the EGR feed out of the head to the valve by a cast exterior pipe. A single serpentine toothed belt drives the Bosch alternator and NC compressor. The steering pump is driven off the intake camshaft.