Pushrod engine
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 OHV design, and is the archetype for most modern pushrod engines. 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.
Three specific problems remain with pushrod engines:
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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, conventional wisdom says that 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 â€" 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 crankshafts, and lightweight valvetrains similar to those used in
NASCAR racing can rev in excess of 8000 rpm.
Because overall power is a 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 incapable of revving. A pushrod engine needs to have a larger displacement than a similarly powered OHC engine. Higher engine RPMs results in more power overall. 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 horsepower, while requiring less torque to do so.
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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.
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Limited valve 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.
In contrast, pushrod engines have specific advantages:
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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.
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Complexity - In some situations OHV can offer reduced complexity.
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.
External links:
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Pushrod (OHV), SOHC and DOHC engine animated diagrams
