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Noise Concerns White Paper

Vehicle Noise Levels and Compression Release Engine Braking


This document is intended to provide the reader with information about commercial vehicle noise and its relationship with compression release engine brakes. This relationship is a subject of community concern and often results in the posting of "No Engine Brake" signs along roads and highways. Data is presented illustrating the relationship between vehicle noise and the condition of the vehicle's exhaust system. This data identifies improperly muffled vehicles as the principal cause of the vehicle noise that concerns communities. This document examines existing regulations that govern vehicle noise levels and presents suggestions for effectively addressing noise concerns at the community level. References are provided for further reading on the subjects of engine braking and vehicle noise.


Compression release engine brakes (referred to hereafter as engine brakes) are the most popular type of supplemental vehicle retarder used in North America. Their function is to turn a power producing diesel engine into a power absorbing air compressor. It does this by quickly opening the exhaust valve near top dead center of the compression stroke. This causes a sudden release of compressed air from an engine cylinder into the exhaust system. This is what causes the characteristic staccato sound of an engine brake in operation. The engine brake is activated only when the driver's foot is off of the accelerator pedal and no fuel is being injected into the cylinder.

It is well known that the stopping power available from a vehicle's service (or wheel) brakes decreases significantly as the brake lining temperature increases. One of the uses of engine brakes on commercial vehicles is to help control vehicle speed on long downgrades. Minimizing the use of the vehicle's service brakes virtually eliminates the likelihood of overheating the brakes and thus helps to avoid dangerous brake fade. Reduced usage of the service brakes on engine brake equipped vehicles also leads to lower maintenance costs through reduced brake lining wear. Vehicles equipped with engine brakes are more efficient and productive to operate. Enhanced driver control and a reduced risk of brake fade also means safer interaction between all of the vehicles operating on public roadways. The overall result of engine brake usage is of significant value to the trucking industry and to the general public as well.

The need for equipping commercial vehicles with engine brakes is greater today than ever before. Vehicle weight and speed limits have been increasing. At the same time the vehicle's natural retarding power has decreased due to reductions in aerodynamic drag and rolling resistance. These improvements are beneficial in terms of vehicle fuel consumption and operating cost. However, they require that more work be done by the service brakes to maintain speeds on long down grades or slow the vehicle. The increased load being placed on vehicle service brakes led to the issuance of an industry practice recommending the use of supplemental retarders [7]. In addition to supplementing the vehicle's service brakes, engine brakes are also being integrated into other vehicle functions such as cruise control, automatically shifted manual transmissions, and the newly introduced collision avoidance systems. These factors are why the majority of heavy-duty vehicles produced in North America today are equipped with engine brakes when delivered from the vehicle manufacturer.


Residents near steep downgrades, highway exits and curves in some communities in North America have expressed concerns about commercial vehicle noise. These concerns frequently identify engine brakes, due to their characteristic sound, as the cause of the objectionable noise. Signs prohibiting engine brake usage have been posted in some communities. The trucking industry is sensitive to these concerns and has studied the issue with regard to both new and in use trucks.

Truck, engine and equipment manufacturer studies have consistently found that improperly muffled vehicles are the root cause of this noise issue. Vehicle operating sound levels have been shown repeatedly to be much higher for vehicles with improper, defective or deteriorated mufflers. The problem is most pronounced on vehicles equipped with "straight stack" exhaust systems (i.e., no muffler). Studies have found that the sound level from "straight stacks" is 16 to 22 dB(A) higher than from original equipment mufflers [1]. Studies have also shown that the operation of an engine brake produces sound levels that are similar to those produced during acceleration on properly muffled vehicles [2].
Figure 1 shows total vehicle sound level data for a typical heavy-duty diesel powered vehicle. Sound levels are measured in 'A' weighted decibels or dB(A). This is a logarithmic scale weighted to the sensitivity of human hearing. Each doubling of a sound source will increase the sound level by 3 dB(A). An 18 dB(A) increase corresponds to a 64 fold increase in the sound source. Additional information on other vehicle/engine combinations is presented in Society of Automotive Engineers (SAE) papers [1] and [2].

Figure1. Typical Heavy-Duty Vehicle Results



Improper, defective or deteriorated mufflers will increase vehicle sound levels over those of properly maintained exhaust systems. The magnitude of the increase though is not as large as that for "straight stacks". A question that can be asked is how prevalent are improperly muffled exhaust systems on commercial vehicles? One survey observed a moderate traffic volume consisting of about 300 trucks per hour traveling on a stretch of Indiana highway. It found 5.3 percent of the trucks did not have a functioning muffler; in fact, 2.4 percent of the vehicles inspected were operating with "straight stacks" installed [1].

From this data one can conclude that residents living near that stretch of highway were on average exposed to 16 vehicles per hour with improperly muffled exhaust systems. These vehicles would be operating beyond acceptable noise levels during acceleration as well as retarding. Overall, this information supports the position that the root cause of objectionable vehicle noise is improperly muffled vehicles.

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