Pushrod engine
From GM Wiki
Template:Mergewith A pushrod engine or overhead valve (OHV) engine is a type of piston engine that places the camshaft in the cylinder block (usually beside and slightly above the crankshaft in a straight engine or directly above the crankshaft in the V of a V engine) and uses pushrods or rods to actuate rocker arms above the cylinder head to actuate the valves. Lifters or tappets reside in the engine block between the camshaft and pushrods.
This contrasts with an overhead cam (OHC) design which places the camshafts above the cylinder head and drives the valves directly or through short rocker arms. In an OHC engine, the camshafts are normally part of the cylinder head assembly, while in an OHV engine the camshaft (rarely more than one) is part of the main engine block assembly.
Pushrod engines are perceived to be "old fashioned" by the modern automotive press. The cause is historical: While both layouts are over 100 years old, the OHV engine came first. OHC engines were developed as more expensive high-performance engines and have largely replaced the pushrod design in countries where cars are taxed based on engine displacement. In 1949, Oldsmobile introduced the Rocket V8. It was the first high-compression overhead valve (OHV) design, and is the archetype for most modern pushrod engines. Currently General Motors is the world's largest pushrod engine producer, as most other manufacturers use OHC designs. Many of General Motors pushrod engines are very reliable and powerful, such as the 3800 Series II V6 Supercharged V6 (240 hp, 280 lb/ft torque and 28 MPG Highway), LS7 Corvette 7.0L V8 Engine (505 hp, 475 lb/ft torque, 24 MPG Highway) and LS4 5.3L DOD V8 (303 hp, 323 ft·lbf torque, 28 MPG Highway.) Currently few pushrod type engines remain in production, a result of the fact that it has become difficult to achieve competitive engine performance with the configuration.
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Limitations
Three specific problems remain with pushrod engines:
- Limited engine speeds or rpm - Pushrod engines have more reciprocating mass, suffer more easily from valve "float", and exhibit a tendency for the pushrods themselves to flex or snap at high engine speeds. Therefore a pushrod engine cannot revolve ("rev") at engine speeds as high as an OHC design. Modern pushrod engines generally rev to 6,000 rpm: compare this to modern OHC engines that can easily rev from 7,000 rpm in average engines to near 20,000 rpm in Formula One racing engines. High-rev pushrod engines can and have also been developed — in 1969, Chevrolet offered a Camaro Z28 with a pushrod V8 that revved to 8,000 rpm. Volvo B18 and B20 engines can rev to more than 7000 rpm. The 2006 Chevrolet Corvette features a 7.0 L engine capable of revving to 7000 rpm. Custom manufactured or modified engines that utilize oversquare piston travel, and lightweight valvetrains similar to those used in NASCAR racing can rev in excess of 8000 rpm.
Because overall power is computed by multiplying torque by revolution speed (HP = (RPM x LB-FT) / 5252), an engine capable of revving higher will produce more power from the same amount of torque than one revving lower. A pushrod engine therefore needs to have a larger displacement to match the power out-put of an OHC engine. A good comparison would be the 3.9L GM 3900 Engine to the 3.0L Honda J-series V6. The smaller (in displacement) Honda Engine produces more power, while requiring less torque to do so.
- Difficulty in using crossflow cylinder heads in straight engine configurations - A few straight pushrod engines have been manufactured with crossflow heads, such as the six cylinder Humber Super Snipe. These engines combined much of the performance of the overhead camshaft with the ease of service of the pushrod, but were more expensive to manufacture than either competing design.
- Limited design flexibility - The biggest benefit of an OHC design is the use of multiple intake and exhaust valves and variable valve timing. Most modern pushrod engines have two valves per cylinder, while many OHC engines use three, four or even five valves per cylinder to achieve greater efficiency and power. Recently, however, GM has begun offering a pushrod V6 with VVT, and Cummins' ISB is a 4-valve pushrod straight-6. For the 2006 model year, General Motors will introduce the Vortec 6200. This is the first mass-produced pushrod engine to feature variable valve timing. The system adjusts both intake and exhaust timing between two settings.
Advantages
In contrast, pushrod engines have specific advantages:
- Smaller overall packaging - Because of the camshaft's location inside the engine block, pushrods are generally more compact than an overhead cam engine of comparable displacement. For example, Ford's 4.6 L OHC modular V8 is larger than the 4.9 L OHV Windsor V8 it replaced and GM's 4.6 L OHC Northstar V8 is slightly taller and wider than GM's larger displacement 5.7 to 7.0 L OHV LS V8. The Ford Ka uses the venerable Kent Crossflow pushrod engine to fit under its low bonnet line.
- Complexity - In some situations OHV can offer reduced complexity.
- Torque - In most situations relative to OHC engines, OHV engines provide more torque as well as a flat torque curve.
1994 Mercedes Indianapolis 500 engine
The Indy 500 race in Indianapolis each year bears some vestige of its original purpose as a proving ground for automobile manufacturers, in that it once gave an advantage in engine displacement to engines based on stock production engines, as distinct from out-and-out racing engines designed from scratch. One factor in identifying production from racing engines was the use of pushrods, rather than the overhead cams used on most modern racing engines; Mercedes-Benz realized before the 1994 race that they could very carefully tailor a purpose-built racing engine using pushrods to meet the requirements of the Indy rules and take advantage of the 'production based' loophole but still design it to be state of the racing art in all other ways, without any of the drawbacks of a real production-based engine. They entered this engine in 1994, and, as expected, dominated the race. After the race, the rules were changed to prevent a recurrence, and the engine became obsolete after just the one race, as Mercedes-Benz knew it would when deciding a victory at Indy was worth it.
