DOC Metal Substrate

How to Size a DOC Metal Substrate for Different Engine Displacement Ranges

When you work with emission systems long enough, you realize something pretty quickly: a DOC that fits one engine almost never fits another. The sizing of a DOC Metal Substrate depends heavily on displacement, but also on exhaust temperature swings, the space you’re given, and how much backpressure the engine can tolerate. There isn’t a perfect formula—just experience, data, and a lot of trial runs.

Below is a practical way engineers usually think about sizing across different displacement ranges.

1. Small Engines (1–3 L): Usually Tight on Space

With smaller engines—light gensets, compact machinery—you don’t get much room to work with. The exhaust flow is low, so the DOC doesn’t need a large diameter, but the substrate has to heat up quickly.

Typical setup for this group:

Higher cell density, usually around 300–400 cpsi

Thin foil to cut down thermal mass

Shorter length, so pressure drop stays manageable

Small engines tend to benefit the most from a metal substrate. It warms up fast, and the catalyst gets active almost immediately after cold start.

2. Mid-Range Engines (3–7 L): The Balancing Group

Engines in this range show up everywhere—from mid-size trucks to general-purpose industrial equipment. The challenge is finding the right middle ground: enough surface area to keep emissions in check, but not so much restriction that it bothers the turbo.

You often see:

200–300 cpsi cell density

Standard foil thickness

A bit more axial length to give the exhaust stream time to react

At this size, DOCs are often paired with DPFs. So temperature distribution becomes important. A DOC Metal Substrate helps because it spreads heat evenly and handles vibration from daily operation better than ceramic units.

3. Large Engines (7–15 L): Flow Volume Drives Everything

Once you’re dealing with heavy trucks, loaders, or bigger generators, the exhaust flow becomes the main constraint. Backpressure matters a lot, and the DOC must survive long hours at high load.

Design usually shifts toward:

Lower cell densities, sometimes 150–200 cpsi

Thicker foil for strength

Larger diameter substrates—or sometimes two bricks in series

Reinforcement to tolerate strong vibration

Ceramic carriers rarely survive this environment. A metal substrate is simply more robust, especially when the equipment runs in rough terrain or sees constant load cycling.

4. Very Large Engines (15 L+): No Standard Solution

Marine diesels, high-capacity generators, and mining engines behave differently from anything on the road. Fuel quality is inconsistent, temperatures swing, and installation space is always awkward.

For engines like these, the DOC is basically custom-built:

Very low cell density (even 50–100 cpsi)

Thick, reinforced foil

Oversized diameter to keep pressure drop reasonable

Multiple metal substrates in one housing

Extra support rings or vibration damping features

In this range, a DOC Metal Substrate isn’t just preferred—it’s almost mandatory. Ceramic bricks crack too easily when exposed to hull vibration, wide temperature variability, or poor fuel.

Practical Rules Engineers Actually Use

After enough projects, a few rules stick with you:

Bigger displacement → bigger inlet area

More flow → lower cpsi

More vibration → thicker foil

Limited space → higher cpsi to compensate

Fast light-off → thin foil and controlled thermal mass

And no matter how perfect the spreadsheet looks, the prototype still needs real-world validation. Engines behave differently on-site—especially off-road engines under transient load.

Choosing a DOC Metal Substrate isn’t about following a universal chart. It’s about understanding the engine’s habits, the environment it works in, and what compromises the application can accept. Displacement gets you started, but sizing is ultimately shaped by testing and experience more than anything else.