What Every Driver Should Know About Engine Braking
Do you often hear drivers complaining about weak engine brakes?
Some drivers claim that their engine brake provides such little retarding power that they have to rely on their service brakes to help slow down the truck. In instructional YouTube videos, you may come across advice suggesting the use of service brakes to supplement supposedly “weak” engine brakes.
Some drivers who offer advice on using engine brakes don’t fully understand how they work themselves. They share misguided information, leading others astray. For example, one driver in a popular video with 520,000 views expressed surprise at the ineffectiveness of the engine brake in a certain truck, wondering if it was due to the muffler system or if the engine brake was inherently weak. In another video with 192,000 views, the same individual warned against letting the engine over-rev, claiming it could damage the engine brake and the engine itself. However, this advice is far from accurate and is not supported by those who truly comprehend engine brakes. Unfortunately, many drivers absorb such information without questioning its validity.
The reality is that engine brakes seem to be less well-understood today than they were in the past. Modern drivers, particularly those with less than five years of experience, may be less technically inclined, often having only driven vehicles with automated transmissions. They might lack a comprehensive understanding of the intricate relationship between gear selection, engine speed, torque, horsepower, and retarding power. With advanced electronic systems doing most of the thinking, drivers have become somewhat disconnected from the fundamentals of engine brake operation, taking this crucial tool for granted, much like crossmembers.
What is an Engine Brake?
Despite its name, an “engine brake” is not a brake in the conventional sense. Rather, it’s a “Compression Release Engine Brake,” a hydraulic system that alters the cylinder valve timing, effectively transforming a power-producing internal combustion engine into an energy-absorbing air compressor. Though commonly known as engine brakes or Jake Brakes, they do not function by conventional braking methods.
When the engine brake is activated, the system modifies each cylinder’s exhaust valve open-and-closed cycles and cuts off the fuel supply to the engine. During the normal operation of a four-stroke diesel engine with the engine brake off, the four strokes are as follows:
- Intake stroke: The piston descends, drawing air into the cylinder through the open intake valve.
- Compression stroke: The piston rises, compressing the air against the closed intake and exhaust valves.
- Combustion stroke: Fuel is injected into the cylinder, and combustion occurs, pushing the piston down, providing energy to power the truck.
- Exhaust stroke: The piston rises again, expelling the exhaust gas from the cylinder through the open exhaust valve.
When the engine brake is activated, the process changes as follows:
- Intake stroke occurs normally.
- Compression stroke occurs normally.
- At the top of the compression stroke, no fuel is injected, and combustion does not take place.
- Simultaneously, at the top of the compression stroke, the exhaust valve briefly opens, releasing the compressed air from the cylinder into the exhaust manifold.
The Effects of Engine Brakes
The effect of these changes is twofold. First, no power is produced, and the engine doesn’t drive the wheels since there was no combustion. Second, the energy required to compress the air in the cylinder comes from the truck’s momentum transmitted through the driveline and crankshaft to the pistons. This effort to compress the air provides resistance to the driveline’s rotation, thereby “retarding” the truck’s motion. For this reason, engine brakes are sometimes referred to as “engine retarders.”
You might recognize the loud popping sound that engine brakes make, which is the result of compressed air escaping from the cylinder through the open exhaust valve. However, this noise is significantly reduced in modern engines thanks to the sound-attenuating properties of the diesel particulate filter.
Now, the perceived reduction in engine brake performance that some drivers experience in modern vehicles is not primarily due to changes in the exhaust system, but rather, it can be attributed to engine speed.
Key Factors Affecting Engine Brake Efficiency
While drivers may complain about weak engine brakes, it’s essential to clarify that there’s no such thing as a “weak” engine brake. However, their performance can be affected by how they are used.
Engine brake retarding power curves for various heavy-duty engine models in North America demonstrate that retarding power increases as engine speed rises. Engineers often use graphs and charts to illustrate this concept. For instance, a graph might display the retarding power, measured in kilowatts per liter of displacement, plotted against engine rpm. The curve generally shows that retarding power is higher at higher engine speeds.
It’s important to note that engine displacement, or the size of the engine’s cylinders, plays a role in the amount of retarding power produced. Larger cylinders can hold more air, resulting in higher retarding power. For example, at 1,200 rpm, an engine might generate around 12 kW/L of retarding power. For a 15-liter engine, this translates to 180 kW (244 horsepower) of retarding power. At 2,200 rpm, the output might jump to around 31 kW/L, or 630 horsepower.
The calibrations of the turbocharger also influence engine brake performance. In retarding mode, the turbocharger pushes more air into the cylinders, increasing the compression ratio and thus providing more retarding power.
It’s essential to recognize that the engine brake operates without consuming any fuel. The electronic controls of the engine brake shut off fuel supply to the engine while it’s in use. This means there is no fuel economy penalty for running the engine brake at higher rpm, such as 2,200 rpm.
Gauging Engine Speed
Considering the issue of weak engine brakes, it’s essential to understand that if your engine brake isn’t providing the desired retarding power, you’re likely running it too slowly. Complaints about engine brake performance are commonly addressed by the original equipment manufacturer (OEM), but investigations sometimes reveal programming errors or a driver’s lack of knowledge on how to operate the engine in retarding mode.
It is not advisable for fleet managers to discourage drivers from running engine brakes at high rpm; in fact, engine brakes are designed to operate effectively at higher rpm. There have been instances where fleets penalized drivers for operating the engine brake at higher rpm, which can be counterproductive. Engineers design engine brakes with criteria that allow them to operate at much higher rpm (up to 3,000 rpm in some cases) for extended periods under full load. This is to ensure their durability and longevity even under maximum overspeed conditions. Thus, engine brakes are designed to last indefinitely at normal operating speeds.
It’s important to clarify that not all engine brakes fall under the Jacobs brand. For specific information about the normal operating speeds of your engine brake model, consult the owner’s manual or reach out to your sales representative.
Customizing Engine Parameters to Enhance Your Engine Brake
One factor contributing to the perception of weak engine brakes is the use of low-engine-rpm downsped drivelines. Engine brakes do not deliver optimal retarding power at low rpm, such as 1,200 rpm. To make the most of their engine brakes, drivers may need to downshift one or two gears to achieve sufficient retarding power.
Ideally, the truck’s cruise control should manage these downshifts when engaged based on the demand for deceleration. However, without active cruise control, the truck may continue accelerating on downhill slopes, leading drivers to rely on service brakes for slowing down.
Fleets may attempt to address this issue by locking out certain functionality on automated transmissions to encourage drivers to use cruise control. However, this can be counterproductive if drivers are not comfortable with the settings.
To improve drivability, it’s crucial for drivers to understand how automated manual transmissions (AMTs) and cruise control work together. Lack of awareness regarding the nuances of the cruise control system could lead drivers to overlook some of its functionality, affecting the performance of the engine brake.
What are High Power Density Engine Brakes?
For fleets hesitant to operate engine brakes at recommended rpm levels, there is an alternative solution to improve performance at low engine rpm: the High Power Density Engine Brake (HPD).
HPD engine brakes were specifically developed for low-rpm engines. Factors like low-rolling-resistance tires and advanced tractor and trailer aerodynamics have reduced the retarding forces that used to aid in slowing down the truck. Coupled with the trend of downspeeding engines and reducing engine displacement, there is an increased demand for supplemental retarding technology.
HPD engine brakes are complex, but in simple terms, they utilize variable valve opening and closing capability to provide two compression release events for every full rotation of the engine crankshaft, compared to just one with a standard engine brake. This nearly doubles the retarding capability of the engine at maximum rpm.
For downsped, low-rpm engines, HPD engine brakes offer comparable performance to standard engine brakes while operating at much lower rpm. Consequently, there’s no need to downshift multiple times to achieve optimal engine brake performance.
Engine brakes are an essential tool for controlling vehicle speed and reducing brake wear. However, if drivers are not well-versed in their proper usage or if fleet settings hinder their effectiveness, drivers may resort to using service brakes more frequently. It’s crucial for fleet managers to review their drivers’ understanding of engine brake systems and address any knowledge gaps. Failure to do so might lead drivers to seek information from fellow drivers on platforms like YouTube or TikTok, where advice might not always be accurate or reliable.
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