IT has taken a long time for the crude first cars to develop into the modern marvels we enjoy today. Along the way, motorists’ expectations and perceptions have changed dramatically.
This was numbingly illustrated to me recently when I was offered the chance to drive a 1931 Ford Model A, one of the best sellers of the late-’20s. It was fun, but so different. The car had more in common with a tractor than with any modern vehicle. The bumpy ride and noisy engine were to be expected, but the unbelievable lowdown torque was a real eye-opener. When the owner told me that a Model A was driven in the ‘30s from sea-level to the cable-way station halfway up Table Mountain in top gear, I was inclined to believe him.
Although it had a four-cylinder engine, the level of engine vibrations varied from the kind that cracks rear-view mirrors to just a pleasant lumbago-numbing zizz, depending on whether the engine was on the over-run or the throttle was wide open.
This was puzzling until I remembered a theory. The loads imposed on the crankshaft by the compression and combustion process opposes the loads imparted by the inertia of the accelerating pistons and con-rods. This means that the intake mixture and burning gases act like a damper to the piston/con-rod movement. Open the throttle and the damper is present; close the throttle and the damper is gone.
This experience prompted me to jot down some of the changes that have taken place with regard to motorists expectations as well as the way cars have been refi ned over the last 125 years. The present article will concentrate on engine vibration control; the next article will deal with other aspects of vehicle design.
The first successful engines were either singles or narrowangle V-twins, so pioneer motorists had to put up with a great deal of vibration from the engine. Designs with more than two cylinders did not sell well until after 1900 because, in most cases, reliability was just a dream and engines with more cylinders were regarded as too troublesome, but by 1910 fourcylinder powerplants were becoming popular. Most designs vibrated much more than modern four-cylinder layouts because their crankshafts were devoid of any balance masses.
The Ford mentioned earlier is an example of such a design. But, by the middle-’30s four balance masses started to appear and by 1970 the change to eight balance masses was in full swing. This was done because refinement became a selling point, but even today there are still some inexpensive cars with only four balance masses on the crankshaft. These engines are not renowned for smoothness. Many modern four-cylinder engines are also equipped with two balance shafts to help create an engine that is almost as smooth as a six. (See panel for explanation.)
HORIZONTALLY-OPPOSED ENGINES
Most people were introduced to these engines by the four-cylinder VW Beetle. These units exhibit the advantages of such a layout. They are inherently smooth and do not need a balance shaft as the only imbalance is a small secondary moment, because the two sets of con-rods in opposing banks are slightly offset to each other.
This results from the fact that they’re usually next to each other on a common crankpin. On a flat-six such as fitted to a Porsche or Subaru, even these moments cancel out with the result that these engines are very smooth.
SIX CYLINDERS
Modern cars are designed with NVH-control in mind, because this has become a strong selling point. (NVH is the abbreviation for noise, vibration, and harshness.) The first really smooth engine, a six, was marketed as far back as 1904 by Rolls-Royce and Napier, but it was soon found that the long crankshaft created enough torsional vibration to break the unit in half.
Napier could not solve the problem, but both Henry Royce and Fred Lanchester invented torsional vibration dampers that were able to reduce the vibrations to a manageable level. These early sixes were in-line units and in such an engine the inertial forces and moments, as well as the firing intervals, are in perfect balance. It is no wonder that the motor industry has produced some memorable straight sixes over the years.
Lancia built the first successful V6 in the early-‘50s, and this layout has recently become very popular because it is compact. Such an engine is not inherently in balance like a straight-six, and there are two ways to tackle the problem. Manufacturers that have a V8 in their line-up usually choose a 90-degree bank angle for their V6 engines so that they can use the same machine tools to produce both engines.
This introduces unequal firing intervals unless the crankpins for opposing pistons are staggered 30 degrees and also results in unbalanced forces and moments that can only be smoothed-out with an engine-speed balance shaft. The better option is to choose a 60-degree bank angle. This reduces the unbalance to levels that can be tamed with clever engine mountings, and also ensures even firing intervals.
MORE THAN SIX CYLINDERS
In the late-’20s the straight-six layout was extended to eight cylinders, and by the middle ’30s straight-eights were all the rage in the USA. Most of them were side-valve units that were renowned for smoothness rather than performance. Some designs lasted until the early-’50s when they were replaced by more performance- orientated V8s.
The first production automotive V8 dates from 1910. It was built in small quantities by De Dion of France and some were sold in the USA, where they caused quite a sensation. Cadillac introduced the first mass-produced V8 in 1914 and sold 13 000 in the first year of production. Ford introduced such an engine into the lowerpriced class in 1932 and today this is the most popular layout for engines over four litres. The overall balance is not as good as in the case of a straight-six, but careful application of balance masses at the right places can result in a very smooth engine.
Enzo Ferrari is said to have been inspired to start building his own engines when he saw a side-valve Packard V12 and we all know how successful that dream was. The Packard, introduced in 1916, was the first practical engine with such a layout. Since then many manufacturers have designed V12 engines for their top-of-therange models. These engines are obviously even smoother than the best six, and hence do not represent a challenge from the balancing point of view. A few V16s have also been built, notably by Cadillac, but most people regard this engine layout being just too complicated.
FOUR-CYLINDER ENGINE BALANCE
There are two reasons why a four-cylinder engine tends to vibrate. One is that the pistons and con-rods are supposed to balance each other as they move up and down the bore, but they don’t. That’s because the upward movements of the pistons are not identical to the downwards movements.
The bottom part of the con-rod moves sideways as the piston descends, so the pistons achieves maximum speed not halfway down the bore, as you would expect, but well before halfway down, when the con-rod makes an angle of 90 degrees with the crankshaft arm. (This point would have coincided with a point halfway down the bore if the con-rod was infinitely long.) The result is that most of the internal forces and moments are in balance except the secondary forces, such as those due to the con-rod’s sideways movement.
These are not in balance, but the resulting vibration can be minimised with two balance shafts, driven by the crankshaft at twice the engine’s speed. These are shafts with protrusions that cause inertial forces when rotated and were invented as long ago as 1904 by Fred Lanchester and used for the first time on a car engine by Mitsubishi in 1975.Many modern engines are fitted with balance shafts.
The rotating big-end journals also have to balance each other and they do, but they are so far apart that the forces involved cause internal torques or moments inside the crankshaft that tend to make the crankshaft shake itself like a wet dog when the throttle is closed. (This is what causes the closedthrottle vibration on the Model A.)
The partial cure is to fit one balance mass near each journal and automotive engineers started doing this in the early- ’40s. This reduced the magnitude of the internal moments, and made four-cylinder engines a lot smoother. A further refinement, introduced in the late- ’70s, was to fit one small balance mass on each side of a journal to balance one-half of each journal. This reduced the internal moments even further and nowadays these eight-balance- mass cranks endow many four-cylinder engines with a further improvement in engine smoothness.
Nevertheless, a four can never be as smooth as a six because each power stroke takes 90 degrees to complete. This means any power stroke has to die out completely before the next one occurs. On more than four cylinders the power strokes overlap, and the more cylinders there are the more overlap there will be.