I was driving out of the mountains yesterday and downshifted (as usual) to help with speed control. RPMs go up of course, sometimes to the same levels as when I’m climbing the other side of the mountain. I was pondering, this type of elevated RPM driving wouldn’t put near the stress on engine internals as the equivalent RPM range when trailer towing/hill climbing, right? What does it do vis-a-vis lubrication requirements? Again, it would be way less stress, right? In my pea brain the engine internals are spinning at the same speed but with way less load.
Stress on Engine When Engine Grade Braking
- Thread starter MrkofXcellence
- Start date
High-RPM, closed-throttle engine braking imparts a different type of stress than typical.
Accelerating an engine creates high cylinder pressure that forces the pistons down. This puts a strong compression load on the connecting rod.
Deceleration is just the opposite. With the throttle closed, manifold vacuum is high. In this scenario, the engine is working to draw air in where there is none to be drawn. This is engine braking. In this situation, connecting rods are experiencing very high tensile loads. The load is trying to pull the piston from the rod and the rod from the crankshaft. It also concentrates a lot of the load on the cap-side of the rod and stresses the rod bolts.
Accelerating an engine creates high cylinder pressure that forces the pistons down. This puts a strong compression load on the connecting rod.
Deceleration is just the opposite. With the throttle closed, manifold vacuum is high. In this scenario, the engine is working to draw air in where there is none to be drawn. This is engine braking. In this situation, connecting rods are experiencing very high tensile loads. The load is trying to pull the piston from the rod and the rod from the crankshaft. It also concentrates a lot of the load on the cap-side of the rod and stresses the rod bolts.
There is a safety issue here that is far more important than additional stress on your engine during these events: Catastrophic brake failure.
It is vital to use engine braking when driving in the mountains. Failure to do so results in overuse or riding of the brakes. Living in Colorado provides plenty opportunities to witness this. So many times up in the mountains, you smell somebodies brakes cooking. (It smells pretty bad.) Many (or most) times, the vehicle has either out of state plates or is a rental,.
Fortunately, modern brake systems as far better than they were 30 years ago and are able to survive the abuse of riding the brakes all the way down the hill.
It is vital to use engine braking when driving in the mountains. Failure to do so results in overuse or riding of the brakes. Living in Colorado provides plenty opportunities to witness this. So many times up in the mountains, you smell somebodies brakes cooking. (It smells pretty bad.) Many (or most) times, the vehicle has either out of state plates or is a rental,.
Fortunately, modern brake systems as far better than they were 30 years ago and are able to survive the abuse of riding the brakes all the way down the hill.
Engine breaking on gas engine, sounds like intake is at zero psi and exhaust is at 14.7 psi. So 14.7 psi pumping loss. Much less than pressure differentials of compression and power strokes when making power.
My truck sees as much as 85 psi exhaust backpressure when engine breaking, 14.7 intake manifold (diesel with VGT). Still only creates ~300 ft*lbs of engine break, which is less than when making power.
My truck sees as much as 85 psi exhaust backpressure when engine breaking, 14.7 intake manifold (diesel with VGT). Still only creates ~300 ft*lbs of engine break, which is less than when making power.
As noted above, the stresses are different when using the engine for braking. There are two main forms of braking in a gas engine, pumping losses and friction. The highest oil temperature I have ever seen in my 3.6L Durango was using the engine as a brake coming down from the Eisenhower tunnel on I-70. The engine ran at 4,000 RPMs for several minutes. The friction caused the oil temperature to exceed 250F, when the normal engine oil temperature is around 190F.
I wouldn't say that is good for the engine, but getting down a long steep grade safely is the priority.
I wouldn't say that is good for the engine, but getting down a long steep grade safely is the priority.
I've lived in the mountains most of my life. There are numerous grades where you had better shift to a lower gear and use engine braking, or you'll likely experience brake fade, and ultimately loss of control of the vehicle. Numerous run-away truck ramps attest to the hazards of mountain driving.
A possible exception are high-performance vehicles which can have faaaaar more effective brakes than the average vehicle.
Yes, shifting to a lower gear and running at high RPM does of course put a load on the engine. But any engine worth owning is designed for it.
On our Dually truck I chose the one with the most effective exhaust brake, as being able to maintain control at 36k GCVW was paramount. It did a great job controlling the load.
When I owned HDT's, the Jacobs brakes were regularly used.
A rule of thumb is to descend any grade in the same gear, and at the same speed, as you would use to climb the grade.
A possible exception are high-performance vehicles which can have faaaaar more effective brakes than the average vehicle.
Yes, shifting to a lower gear and running at high RPM does of course put a load on the engine. But any engine worth owning is designed for it.
On our Dually truck I chose the one with the most effective exhaust brake, as being able to maintain control at 36k GCVW was paramount. It did a great job controlling the load.
When I owned HDT's, the Jacobs brakes were regularly used.
A rule of thumb is to descend any grade in the same gear, and at the same speed, as you would use to climb the grade.
That's spot on, but some people haven't gotten the memo.
And now there's modern cruise control ALA Mercedes and I'm sure others. Automatic braking will use the brakes to keep that set speed no matter what! I hate it!
The cruise control gets turned off immediately, and I'll use the brakes as is appropriate for the descent. I would never ride the brakes myself, but cruise control has absolutely no concern for that.
The cruise control gets turned off immediately, and I'll use the brakes as is appropriate for the descent. I would never ride the brakes myself, but cruise control has absolutely no concern for that.
I used to know some numbers …
Thrown rods are apt to happen at peak rpm when the foot is taken off the gas because as mentioned above it shifts the rod forces from compression to tension.
This may be the reason that an ECU allows higher redline rpm in gear than when in neutral. I’ve had 2 v6 which had something like a 4k neutral redline.
And this doesn’t bother me a bit. Engine brake during long descents, all day long.
Thrown rods are apt to happen at peak rpm when the foot is taken off the gas because as mentioned above it shifts the rod forces from compression to tension.
This may be the reason that an ECU allows higher redline rpm in gear than when in neutral. I’ve had 2 v6 which had something like a 4k neutral redline.
And this doesn’t bother me a bit. Engine brake during long descents, all day long.
I always downshift vs. excessive brake use. I find myself wanting to “save them.”. Just using them to put down a little excessive speed, I consider the engine braking to be the primary speed control on long downgrades.
So clearly there is stress being put on the lubricant.