The first stop on a discovery mission around secondary braking systems starts with head of product at Volvo Trucks, John Comer. He states: “There are three types of auxiliary retardation: engine brakes, gearbox retarders and there was a Voith system many years ago called a PriTarder, which was an impeller inside the cooling system to slow down the crank.”
Volvo chose to go with an engine brake as its primary option, the current version of which is VEB+. “It was the change in engine design in the early ‘90s that allowed us to do that,” Comer says. “When we designed the D12 engine for FH originally, for the launch in 1993, we went to a single-piece head with an overhead cam,” he says. Two additional lobes were then added to that overhead cam which, when engaged, opened and closed the exhaust valves as the engine brake was switched on and off. “You’re basically closing the cylinder and creating a compressor, which slows or stalls the engine,” he explains.
The next step on from this came in 2001, when disc brakes with EBS were introduced to Volvo’s FH and FM Version 2 truck range. “Originally VEB was switched on the dashboard, but from Version 2 FH it has been a stalk function, which means it can have several graduations,” Comer continues. “With the launch of the I-Shift gearbox and the integrated CAN-Bus, you’ve got the gearbox, engine and engine brake all speaking to each other.” That opened the door to blended braking, where service and secondary brakes work in conjunction. “So, for example, if you press the B Mode button at the end of the stalk, the engine will look to drop gears, because the engine brake is most efficient at high revs.”
Today’s VEB+ is simply a reworking of that original engine brake, but with an additional hydraulic circuit to engage the cam, making it much more powerful.
“Volvo does have a retarder, made by Voith and integrated with I-Shift, but take up is very low in general applications,” Comer says. “For example, a retarder will give you much more retardation, but as soon as it gets to 100ºC, 600kW drops to 400kW, while an engine brake performs very well at high temperatures and saves 150kg in unladen weight.” He agrees that retarders do have their place, however. “The beauty of a retarder is it works at low engine revs,” Comer adds. “So for heavy haulage operations or in continuous hilly conditions, we will discuss the retarder every time.”
DIFFERENT PRIORITIES
Scania, meanwhile, has long been known for its retarder, although the good old-fashioned exhaust brake was a mainstay for many years, too. “The exhaust brake is essentially a butterfly in the exhaust,” says pre-sales technical manager Phil Rootham. “This allows us to slow the vehicle down with a gear engaged, by generating back pressure through the engine.” (Its 13-litre engine model is above, and is shown fitted with gearbox and silencer on p30.)
Next on the list in terms of braking power is what Scania calls a compression release brake (CRB), its variation on the theme of engine brakes. “This works on the valves, limiting the exhaust to generate compression in the engine, which again slows the powertrain down.”
Top of the list, however, comes that famous Scania retarder. “What our retarder does is apply hydraulic pressure to the output shaft of the gearbox,” Rootham tells us. “So again, the prop shaft is being driven by the wheels, and we’re hydraulically applying pressure to that to slow it down.” Retarders can also be geared for specific applications. “Where retarders work really well is in long haulage operations, where you’ve got road speed,” Rootham adds. “The higher the vehicle speed, the greater efficiency we get.”
Historically, Scania offered two levels of retarder, one at 4,100Nm and a second at 3,500Nm. The difference was the gearing – increase the ratio and you improve the performance. “The problem is, you also generate more drag,” Rootham tells us, “so that comes with a penalty in fuel consumption.” Because of this, the higher-force unit was considered as a construction retarder, because it supplied braking effort at low speeds. The lower-force version was lower-geared and therefore generated less drag, but was still effective enough for the higher speeds of distance applications.
To simplify matters, Scania came up with the nattily named R4100D, which has a clutch – the D standing for ‘disengageable’. This meant the retarder was completely disengaged from the gearbox when not in use, removing the drag penalty, so only one version needed to be offered for both scenarios. Throw the new generation of gearboxes into the mix and now the performance has risen as well, and the R4100D is available to all.
Like Volvo, Scania also blends its braking systems for maximum efficiency. In essence, this means thinking about what drivers are asking the vehicle for when they depress the brake pedal. “Generally the answer is foundation brakes, because they’re closest to the wheel and fastest to react,” Rootham says. “So we cover the initial retardation with foundation brakes, then blend those off and blend in the retarder.” All the driver feels is the vehicle braking as required, but behind the scenes use of the retarder is automatically being maximised to reduce wear on the foundation brake linings.
Even Scania, however, is happy to point out that a retarder isn’t always the answer. One big difference between that and a CRB is weight: specify a retarder and you lose 100kg of payload; go for the CRB, and the penalty is only 7kg. “So if you move into the world of construction, you’ve got tippers, where we’re talking about site use, high engine speed, and steep hills up and down,” Rootham points out. “There, CRB really comes into its own because it doesn’t affect payload, yet it still offers performance at those really slow speeds.”
As is the case with many components, most OEMs don’t actually build their own engine brakes and retarders, instead sourcing them from one of a few specialist suppliers. Volvo’s retarder is made by German engineering giant Voith, but there are several others, including Pacbrake in the US and Eaton Corporation. ZF produces what it calls an Intarder – again a gearbox retarder – which it says is unique in that it continues to apply braking force as the driver changes gear.
In the coach world, meanwhile, there was the Telma retarder, a 1980s French design which worked using an electromagnet – still relevant today because it used the heat produced to generate current, which it fed back to the battery, in a similar manner as the regenerating brakes seen on modern electric commercial vehicles.
Perhaps one of the best-known companies in this field, however, is Jacobs Vehicle Systems, manufacturer of the ‘Jake Brake’. The original Jake Brake was invented by a man called Clessie Cummins, founder of what is now Cummins Inc, so there’s a certain serendipity in the fact that Cummins Inc bought Jacobs Vehicle Systems in early 2022.
Gabe Roberts is director of product development, valvetrain technologies at Cummins Engine Components, having first been with Jacobs Vehicle Systems for many years. Around 75-80% of all engine brakes in service were designed and developed by his team, although a lot are rebranded by customers using their own terminology.
Starting with the least effective, first on its list is the exhaust brake or exhaust throttle mentioned above, use of which is still surprisingly common. It is particularly popular in countries like India, where engine technology has not yet advanced enough for other options, and a low-cost solution is preferred. To increase performance, an exhaust brake can be combined with a bleeder brake. “A bleeder brake just holds the exhaust valve open during the entire stroke,” Roberts explains. “This means you get high back pressure during the compression stroke as well as the exhaust stroke, which slows the piston down.” Simple, cheap and it bolts directly on to the engine.
COMPRESSION RELEASE
The main type of engine brake today uses compression release technology. It adds a third rocker arm to each engine cylinder, so Cummins calls it the DRB – dedicated rocker cam brake (the inset shows the brake activated, with actuator lowered). Roberts says: “It’s a cam-driven brake, and we open it at two times. When the engine brake is turned on via a solenoid, small pistons come out of the rocker to give very precise valve lift events.”
When the piston reaches the top of the stroke – in the compression phase – that slight opening of the exhaust valve allows the compressed gas to escape. “The amount of force, or of torque, applied during the expansive stroke is significantly reduced – it’s basically none,” Roberts says. “This creates a form of negative torque on the engine.” Because the piston isn’t then returning any force to the crankshaft on the following expansion stroke, the vehicle slows.
Fitted to all six cylinders, different levels of braking can be achieved. “The lowest setting is actually only using three cylinders, so only three engine brakes are turned on,” Roberts tells us. “The second, medium setting uses all six, but we open the turbo waste gate (or variable geometry) to restrict the performance. The high mode,” he continues, “is usually the full capability of the engine brake at that rpm.” Add a gearbox retarder into the mix as well, and two levels of braking are enabled.
The DRB is Cummins’ flagship for heavy duty engines, but the manufacturer produces other forms of compression release brake, too. The best example is probably the Lost Motion Brake. This is similar to the DRB, but the brake is part of the existing exhaust rocker. This means that one rocker is doing two jobs: the exhaust event when the brake is off, and both the exhaust and braking events when the brake is on.
“The cam actually has a section which is normally hidden, and we call that ‘lost motion’,” Roberts explains. When the brake is turned on, that part of the cam is revealed. This lobe activates the small piston on the rocker, which enables the partial opening of the exhaust valve and creates the braking event.
“We put the two technologies into one arm because it’s cheaper for the customer,” Roberts tells us. “It’s good for medium-duty engines where you don’t have enough room for a third rocker on every cylinder.” Also called the Integrated Rocker Brake (IRB), this configuration is particularly popular in places like China, in no small part because of the lower cost.