Engine Speed Related Vibrations

After your road test, park the vehicle and bring the engine speed up to the same speed that it was vibrating at on the road test. If the vibration is present, record the Vibration RPM and Engine RPM on the vibration worksheet. Mark the point on the vibration worksheet where your Vibration RPM and your Engine RPM readings intersect. The point of intersection should be on or near a vibration line on the vibration worksheet. Each line is identified with an abbreviation like E1. E1 represents a first order engine speed related vibration.

An engine speed related vibration would usually always occur at the same engine RPM speed regardless of vehicle speed. This type of vibration can usually be detected with the vehicle stopped, although sometimes it may only show up under a load. 

There are two categories of engine vibrations:

• Abnormal Vibrations - Vibrations caused by a component rotating the same speed as the engine's crankshaft, a belt-driven accessory, or a gear-driven component that is out-of-round, out-of-balance.  Abnormal engine vibrations can also be caused by piston slap, excessive valve lash, excessive bearing clearances, deformation of parts under stress, crankshaft and camshaft deformations, etc.
• Normal Vibrations - Vibrations caused by normal engine operating characteristics such as: cylinders firing events, rocking motions, crankshaft deflection, exhaust pulsations, etc.  All engines have vibrations which are a normal part of the engine running; however, they should not be felt in the passenger compartment.  Click here to see which vibrations are normal.  If the source of the vibration is a normal engine operating characteristic, do not attempt engine repairs because there is nothing wrong.  Instead, the transfer path for the vibration must be repaired.

Select the engine vibration order you want to diagnose.

• First Order Engine Speed Related Vibrations, (E1 on the vibration worksheet)  First order engine speed related vibrations cause one shake or disturbance for each revolution of the engine's crankshaft.
• 1.5 Order Engine Speed Related Vibrations, (E1.5 on the vibration worksheet)  1.5 order engine speed related vibrations cause 1.5 shake or disturbance for each revolution of the engine's crankshaft.
• Second Order Engine Speed Related Vibrations, (E2 on the vibration worksheet)  Second order engine speed related vibrations cause two shakes or disturbances for each revolution of the engine's crankshaft.
• 2.5 Order Engine Speed Related Vibrations, (E2.5 on the vibration worksheet) Second order engine speed related vibrations cause 2.5 shakes or disturbances for each revolution of the engine's crankshaft.
• Third Order Engine Speed Related Vibrations, (E3 on the vibration worksheet)  Third order engine speed related vibrations cause three shakes or disturbances for each revolution of the engine's crankshaft.
• Fourth Order Engine Speed Related Vibrations, (E4 on the vibration worksheet)  Fourth order engine speed related vibrations cause four shakes or disturbances for each revolution of the engine's crankshaft.
• Fifth Order Engine Speed Related Vibrations, (E5 on the vibration worksheet)  Fifth order engine speed related vibrations cause five shakes or disturbances for each revolution of the engine's crankshaft.
• Sixth Order Engine Speed Related Vibrations, (E6 on the vibration worksheet)  Sixth order engine speed related vibrations cause six shakes or disturbances for each revolution of the engine's crankshaft.
• Other Order Engine Speed Related Vibrations  Other order engine speed related vibrations cause multiple shakes or disturbances for each revolution of the engine's crankshaft.
• Belt-Driven Accessory Speed Related Vibrations

Normal Engine Vibrations Chart

 Normal Firing Frequency

On four-stroke engines, each cylinder is fired once for every two revolutions of the crankshaft.  Each cylinder firing will cause the crankshaft will accelerate and decelerate for each firing event causing a disturbance or motion.  After ignition has taken place, a piston moving downward in a cylinder of an engine running at 2000 RPM with a 4.0 inch stroke, has a maximum piston speed of 11.0 meters per second (24.6 MPH). This means that on a 8-cylinder engine there will be four normal firing pulses per revolution of the crankshaft; a 6-cylinder engine will be have three normal firing pulses per revolution of the crankshaft, a 4-cylinder engine will be have two normal firing pulses per revolution of the crankshaft, etc.  Each firing cylinder will also have an exhaust pulse which can cause a vibration in the exhaust system, the vibration frequency of the exhaust pulses will match normal engine firing frequency.  Normal firing frequency = number of cylinders / 2.  On even number of cylinder engines, there can be a torque sensitive Normal Crankshaft Deflection which also matches this frequency.

Normal Crankshaft Deflection

Normal crankshaft deflection can occur for a variety of reasons, the most common are listed below:

• Torsional Forces - Torsional (Twisting) forces in a crankshaft can be the result of:

  • Gas Forces - When the air-fuel mixture inside a cylinder is ignited, the combustion creates an extremely rapid rise in cylinder gas pressure known as a torque spike. This pressure applied to the top of the piston, applies a force to the crankshaft through the connecting rod. Each torque spike is like a hammer blow. In fact, it hits with sufficient intensity that it not only causes the crankshaft to turn, it actually deforms and twists it (Tangential Force). (After ignition has taken place, a piston moving downward in a cylinder of an engine running at 2000 RPM with a 4.0 inch stroke, has a maximum piston speed of 11.0 meters per second (24.6 MPH)). This twisting action and the resulting rebound (as the crank arm snaps back in the opposite direction) is known as torsional harmonic vibration. If not adequately controlled with a vibration damper, torsional vibration causes rapid main bearing and main journal wear and possible crankshaft breakage.

  • Inertia Forces - As the crankshaft rotates, each connecting rod bearing throw has an inertial force which causes it to deflect outwards (Centrifugal Force).  This outward force deforms the crankshaft.  The amount of deflection is proportional to the mass of the throw, connecting rod, piston, etc. and the rotational speed of the crankshaft.  (Inertia is defined as: the property of a body by which it remains at rest or continues moving in a straight line unless acted upon by a directional force)

• Longitudinal Forces - Longitudinal forces (lengthwise forces on the crankshaft). Significant crankshaft bending occurs on engines with small numbers of cylinders.  The bending is the result of the small number of firing pulses per revolution of the crankshaft.  Engines with small numbers of cylinders require a large mass flywheel to maintain rotational torque.  The combination of a large mass flywheel and crankshaft bending forces causes a second order crankshaft speed related vibration.  Think of this type of force as causing the crankshaft length to increase and decrease two times per revolution of the crankshaft.

• Vertical Forces - Vertical Forces (Up and down forces) - Caused by unbalanced inertia forces in the engine.

• Lateral Forces - Lateral Forces (side-to-side forces) - Caused by unbalanced inertia forces in the engine.

Normal Rocking Motion

Normal rocking motion can occur for a variety of reasons, the most common are listed below:

• Combustion Side Force - Normal rocking motion occurs on most engines because of the combustion side force applied to the pistons.  Most pistons have an offset wristpin bore which is designed to reduce piston slap as the connecting rod journal passes top-dead-center.  The offset wristpin bore causes the combustion force to both act downward on the piston and slightly toward the major thrust surface of the cylinder wall.  The sideward force causes a lateral rocking motion. Some manufacturers use a counterbalance shaft(s) which rotates the same speed as the crankshaft, but in reverse direction in an attempt to counterbalance the rocking motion in their engines

• Odd Number of Cylinders - Normal rocking motion (from moments of inertia) also occurs on engines (V-6 and V-10 engines) which have an odd number of cylinders firing per revolution of the crankshaft.  Even-fire V-6 engines have two cylinders on the left bank of the engine and one cylinder on the right bank will fire on the first revolution of the crankshaft.  On the next revolution, two cylinders on the right bank and one cylinder on the left bank will fire.  Although the engine's crankshaft is balanced to correct first order vibrations, the firing of two cylinders on one bank and only one cylinder on the other bank causes two rocking motions of the engine per revolution of the crankshaft (2.0 order vibration). V-10 engines have a similar problem which causes three rocking motions verses two rocking motions within the engine for each revolution of the crankshaft (1.5 order vibration).  Some manufacturers use a counterbalance shaft which rotates the same speed as the crankshaft, but in reverse direction in an attempt to counterbalance the rocking motion in their engines.

Normal Free Moments of Inertia

In physics, a moment of inertia is a measure of resistance to angular acceleration; in other words a measure of resistance to changes in the angular (rotational) speed of the crankshaft.  As an engine's crankshaft rotates, there are periods where these resistive moments occur.  Each engine configuration has different positions where the resistive moments can occur.  Each free moment can cause a low amplitude vibration which should not be felt in the passenger compartment. 

Piston Movement

As a piston moves up in its bore, it has to come to a complete stop at the top of the bore.  Next, the piston reverses directions and moves to the bottom of the bore and stops.  The stopping of each piston causes a slight motion or low amplitude vibration.  After ignition has taken place, a piston moving downward in a cylinder of an engine running at 2000 RPM with a 4.0 inch stroke, has a maximum piston speed of 11.0 meters per second (24.6 MPH).

The reciprocating weight of each piston and connecting rod and their inertial force is supposed to be counterbalanced by the crankshaft counterweights, but it is rarely perfectly counterbalanced, (even on balanced race engines) especially on mass produced high production engines.   A balanced engine has pistons that all weigh the same amount, connecting rods that all weigh the same amount, and bearings that all weigh the same amount, etc.  The crankshaft counterbalance weight has to be fine tuned to perfectly offset the weight of the corresponding piston, connecting rod, and bearing.  The dynamic weight of the piston can vary during the combustion cycle due to the forces acting on the piston during the intake, compression, power, and exhaust strokes.  These variable forces can contribute to an engine vibration which cannot be counterbalanced.

The number of pistons causing this motion per revolution is half the number of engine cylinders.  Each piston comes to a complete stop twice per crankshaft revolution. Even number of cylinder engines (4, 6, 8, 10, 12) have companion cylinders which causes two pistons to move up and down in their bore at the same time. Odd number of cylinder engines (3, 5) do not have companion cylinders, so each piston to moves up and down in its bore at a different time.  The resulting order of vibration matches the number of pistons in the engine.  For example a 4-cylinder engine will have a normal (very low amplitude) fourth order vibration.

Single Cylinder Misfire

• On four-stroke engines, each cylinder is fired once for every two revolutions of the crankshaft.  A single cylinder misfire would cause one disturbance every two revolutions of the crankshaft.  This is a half-order vibration.  This type of vibration will be accompanied by some sort of engine drivability issue and possible a lack of power from the engine. 

• The camshaft also rotates at half of the crankshaft speed.  A camshaft with bad bearings also can cause a 1/2 order vibration.

Bank-to-Bank EGR/Fuel Variation

A bank-to-bank EGR or air-fuel ratio variation can cause the cylinders on one bank of a V or opposed cylinder configuration engine to misfire or half the cylinders on each bank depending upon intake plenum/manifold design.  This would kill half the engine's cylinders resulting in a vibration which equals the number of engine cylinders divided by 4.  A V-8 engine with a bank-to-bank EGR or air-fuel ratio variation problem would have a second order vibration.

This page was last modified Monday, June 16, 2008 07:33:02 PM

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