So much is written about diesel-engine emissions these days that nobody could
fail to realize that the exhaust system’s chief role in life is to deal with the residue of the combustion process. Those nasty gases and particulates have to be sent packing.

But the exhaust system also controls noise, a job that’s likely to become more
important in the future as North American jurisdictions begin to catch up with
Europe in regulating the sounds trucks make. More and more municipalities are
posting “No Engine Brake” signs, which led one muffler maker to work with an
engine-brake manufacturer in developing a solution (The Donaldson ‘Silent Partner’).

As well, various bits of piping have to be accommodated by the vehicle’s body and frame.

The easy flow of exhaust gases is critically important, and it only takes a few too many elbows or silencing baffles to restrict that flow to a point where the engine is working harder than it should to purge those gases. When that happens, power and fuel economy will suffer, and ultimately engine life too.

Truck makers reach the best compromise possible in their standard exhaust system, but you may have a special need, or you may simply want lower restriction in hopes of improving fuel use or performance. The truck OEM may have options that suit you, or you may visit an independent shop for a custom design.

Especially in the latter case, you would do well to understand the basics of
exhaust system design first.

What’s Restriction?
Getting rid of a diesel’s combustion waste efficiently demands that the exhaust
system – from the turbo outlet on back – imposes as few restrictions as possible. Those restrictions are collectively called ‘back pressure’, measured in
inches of water or mercury. The measurement changes from engine model to engine model, and will change again as the exhaust system itself is modified. A typical measurement – and maximum – is three inches of mercury.

What happens when you’ve got more than the engine’s limit? As we said, the piston takes on more work as the engine pumps harder to sweep the cylinder clean after combustion. If the engine’s working harder – to do the same amount of work – it’s using more energy to create the power that you’re asking for. So fuel economy suffers.

As well, various internal temperatures go up, combustion suffers, and now you’re over-fueling the engine, maybe creating smoke too. Soon you get carbon build-up on the injectors and on the inside of exhaust components. With fouled injectors you get progressively less efficient combustion and with a fouled muffler you’re just increasing restriction again. You’ll see a performance loss within three or four months. In fact, when mechanics are faced with a low-power complaint, they usually look first for signs of this kind of restriction.

A larger muffler – or none at all for those who choose the sometimes illegal straight-pipe option – will reduce back pressure while also being more effective in controlling noise. Back pressure will also be chopped by minimizing the bends and elbows of the piping.

In fact, it’s probably not worth the significant expense of re-designing your exhaust
system to minimize back pressure. You can’t make enough difference, even if you manage to reduce back pressure from three inches of mercury to two, for example. That’s a huge reduction, but in performance terms you’ll only notice the change if your driving style is flat out in every gear at every moment. For the vast majority of truckers, the difference will be invisible.

Limiting Noise
The science of noise is complicated, but there are two key points: one, the human ear is very sensitive, capable of hearing noises at levels from 0 to 120 decibels
(dBA); and two, that’s what’s called a ‘logarithmic’ progression that actually
represents an increase of one billion times. A billion.

As with many other specs, trucks here in Canada are largely built to U.S. noise
rules, though beware that noise regs can also fall under provincial and municipal control. U.S. standards say a new truck cannot exceed 80 dBA at a 50-ft-away drive-by. Under 35 mph, it must not exceed 83 dBA in any operating mode, and over 35 mph it cannot go beyond 87 dBA.

But a typical truck with straight stacks, decelerating with the engine brake on, will produce a noise measured at over 101 dBA. In the same mode, a truck with an
inexpensive aftermarket muffler will typically produce as much as 90 dBA, and with a high-quality OEM silencer it will be around 84 dBA.

Note that a truck operating at 100 dBA is twice as loud as the one at 90 dBA!
Normal conversation, by comparison, is 70 dBA, a private office is 50 dBA, and
a soft whisper at five feet is 35 dBA.

It’s a matter of driver comfort as much as the public’s, of course, because
excessive noise can make driving life miserable. A quality muffler means an
improved driver environment which should bring less fatigue.

An interesting new twist on silencing is the development of a so-called ‘active’
muffler, though there’s no indication that it will come to market soon. It’s a
microprocessor-controlled device that ‘reads’ the engine’s noise and then creates so-called ‘anti-noise’ electronically to cancel out the original sound using microphones and speakers. One advantage is that there’s no muffler and thus less back pressure.

Aftertreatment
Only a few medium-duty diesels, and no heavies, presently require exhaust
aftertreatment by way of expensive car-like catalytic converters to meet particulate emissions regulations (see notes below).

But with much more stringent regs coming in 2002, both medium- and heavy-duty
engines will almost definitely need some new sort of exhaust aftertreatment on
board. Diesels will likely be equipped with exhaust-gas recirculation devices
which will not only cost more but will also increase corrosion, oxidation, and acidic contamination of the engine’s lube oil.

It remains to be seen exactly how this will play out, but one thing’s for certain: truck exhaust systems will only get more complex.

Note: the paragraphs above were written in 2000, and exhaust systems have indeed become more complex. Caterpillar ACERT heavy-duty engines introduced to the
market in 2003 have an oxidation catalyst as aftertreatment. And the 2007 emissions challenge will see all engine makers addinhg a diesel particulate filter.