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Why Do Diesel Engines Last So Long?

By Featured Writer, Mike McGlothlin

 

What do the freight, construction, agriculture, marine, military, commuter transport, power generation, RV, and emergency industries all have in common? They rely almost entirely on diesel propulsion. Along with producing tremendous amounts of torque (which helps get heavy cargo moving), diesel engines are highly efficient under load and typically last two to four times longer than a comparable gasoline-fired engine. But exactly why do diesel engines last so long? From their burly parts and low-speed operation to the type of crude oil-derived fuel they burn, we’re putting the permanence of compression-ignition power plants under the microscope this time.

 

Low RPM Operation

One of the most distinguishable characteristics between a diesel and a gasoline engine is the rpm they register on the tachometer. It’s relatively uncommon to come across a diesel engine that revs beyond 4,000 rpm, and it’s extremely rare to find a diesel that operates at 5,000 rpm, but most gasoline engines make their peak horsepower at this engine speed, if not higher. So why are diesels such low-rpm operators? Thanks to their high compression ratios they can produce gobs of torque at low engine speed, but they can also make sufficient horsepower at low rpm. As a general rule of thumb, an engine turning less rpm is inherently going to last longer than one that’s regularly spinning 3,000, 4,000, or 5,000 rpm. More revolutions typically ensures quicker wear rates.

 

Diesel’s Built-In Safeguard: Long Stroke

Part of a diesel engine’s mechanical makeup is a crankshaft with a considerable stroke length. With most diesels being under-square (i.e. their stroke exceeds their cylinder bore diameter), big torque is on the table at very low rpm. And although diesel engines produce abundant amounts of cylinder pressure to accomplish this, their long stroke allows the pistons to get away from the immense pressure present in the cylinders, as well as the heat from combustion. After all, excessive cylinder pressure can crack pistons, bend connecting rods, and damage crankshafts. In the diesel aftermarket, this lengthy stroke is why factory pistons (and rotating assemblies) tend to survive radical advancements in injection timing, advancements which increase cylinder pressure at top dead center.

 

Easy Does It

Most commercial diesel engines achieve their peak torque output right off idle, usually between 1,100 and 1,600 rpm. These low-speed engines also produce their peak horsepower ratings soon after, and some are even governed as low as 2,100 rpm. The other factor in a diesel’s low-rpm, long-life equation is its robust, heavy-duty construction. Depending on the application, hundreds of pounds can exist in a diesel’s rotating assembly alone (crankshaft, connecting rods, and pistons). A rotating mass that’s this heavy is best left turning at low speed rather than high speed. The combination of low rpm and anvil-like internal hard parts make for a combination that can reward its owner with a million miles or tens of thousands of hours of service.

 

Burly Internal Components

As we’ve mentioned, one benefit of a diesel engine’s high compression ratio is its exceptional torque production at low rpm. But the stress created with copious amounts of low-rpm torque (namely the aforementioned term coined “cylinder pressure”) would destroy a rotating assembly if special care wasn’t taken to ensure its strength was uncompromising. For this reason, diesel engines are often overengineered by the manufacturer. Forged-steel crankshafts, connecting rods and camshafts are all normal here, with induction hardening, Tufftride fatigue strength treatment and thermal barriers even getting the nod from some OEM’s—all in an effort to make these vital pieces last. That, and their physical size is immense in comparison to what you’d find in a typical gasoline engine. Larger bearings with more surface area and more clearance for optimum oil flow also play into a diesel’s ever-lasting nature.

 

Rigid Blocks

Foundationally, most diesel engines employ cast-iron blocks and cylinder heads. More specifically, gray iron alloy is the preferred material due to its cost and stiffness properties (stiffness being prioritized over tensile strength in this case). In some applications, a common one being Ford’s 6.7L Power Stroke diesel V-8 that now turns out 1,200 lb-ft of torque, compacted graphite iron (CGI) is used—a material that is both stronger and lighter than cast iron. Deep-skirt crankcase designs with thick bulkhead sections, large diameter main cap and cylinder head fasteners, and even block stiffeners are fairly standard hardware on medium-duty and heavy-duty diesel engines.

 

Sleeved Blocks

Another point of strength when considering why diesel engines last so long is their use of cylinder liners (also known as sleeves). Many medium and heavy-duty engines use either dry cylinder sleeves or wet cylinder sleeves, the latter of which uses engine coolant to rapidly transfer combustion heat away from the cylinders. As you might’ve already concluded, an engine that sees less heat (i.e. stress) is bound to last longer. Due to its nodular graphite makeup, most cylinder sleeves are made from ductile iron (we’ll note that ductile iron is considerably less brittle than cast iron). On parent bore, cast iron blocks (those not equipped with serviceable cylinder liners), induction hardening is sometimes employed to guard against wear from piston ring travel, such as in the Duramax V-8 found in Chevrolet Silverado and GMC Sierra HD’s.

 

High Quality Pistons

Forced to deal with the flame front within each cylinder, you can bet the pistons in a diesel engine are capable of withstanding countless heat cycles and plenty of abuse. Different piston materials are utilized depending on the engine application, but cast-aluminum is common, as is forged-steel (shown). Some medium and heavy-duty diesel engines even employ two-piece pistons. Direct injection pistons of the Mexican Hat design variety (with the combustion area recessed in the top of the piston) are most common, although indirect injection pistons (along with their unique pre-chamber combustion systems) are still used in smaller diesel applications. To hold up to the cylinder pressure, shock load, and combustion temperatures they see, many pistons are treated to thermal barrier coatings to reduce heat transfer and/or dry film lubricant coatings to reduce cylinder wall scuffing, and it’s not uncommon for compression rings to receive anti-wear and corrosion-resistance treatments at the OEM level.

 

Piston Cooling Jets, Huge Oil Capacity, And Gear-Driven Components

Speaking of pistons, you’d be hard-pressed to find a diesel engine that didn’t benefit from piston cooling jets (also known as piston oil squirters). Piston cooling jets ensure the bottom side of each piston is constantly bathed in engine oil. And on the subject of oil, most diesel engines enjoy a much higher oil system capacity than their gasoline counterparts, which is part of the reason why oil change intervals can be so long on diesels. Adding to the durability factor are gear-driven components rather than timing belts (which can break) or timing chains (which can stretch). That, and most gear-driven diesel engines boast helical gears for superior strength and quiet operation.

 

Other Longevity-Minded Hard Parts

In most light-duty or automotive applications, one thermostat is employed in an engine’s cooling system. This means an overheat scenario is likely if the thermostat fails. In many diesel applications, more than one thermostat is used. This means that in the event of a thermostat failure there will still be coolant flow. Overheating is a major cause of engine failures, be they gasoline, diesel, automotive, or industrial. Additional points of durability for most diesel engines include solid roller lifters, oversize fasteners (such as large diameter main cap bolts), or the use of six head bolts per cylinder.

 

Diesel Fuel Is A Combustible Lubricant

The type of fuel burned is a key reason diesel engines last as long as they do—especially when compared to gasoline engines. Gasoline’s chemical makeup makes it very similar to a solvent. On the other hand, diesel fuel (which like gasoline also comes from crude oil) enjoys a composition that makes it more of a light oil, with good lubrication qualities. So while gasoline is busy cleaning and washing down the engine’s cylinder walls (and doing the piston rings zero favors), diesel fuel is providing a light layer of oil between the piston, piston rings and the cylinder wall. This additional lubrication, combined with low rpm operation results in less friction, and ultimately more longevity in the diesel application.

 

Compression Ignition

Thanks to a diesel engine’s high compression ratio cramming loads of air into each cylinder the becomes super-heated. Then with the addition of finely atomized droplets of diesel fuel ignition takes place without any need for ignition aids (such as spark plugs in a gasoline engine). Without the need for components like spark plugs, which often require replacing every 100,000 miles if not sooner, you benefit from having one less point of wear on a diesel engine. And while the fuel injectors themselves will eventually require an overhaul, 200,000 miles seems to be the minimum lifespan they enjoy in a properly maintained diesel engine.

 

Diesel Engines Run Cooler

In general, diesel engines enjoy cooler operating temperatures than their gasoline counterparts. Leaner air/fuel ratios, a lower auto ignition temperature, and the components used to control coolant, oil, intake air, and exhaust heat all help account for this. On the air/fuel front, diesels can run anywhere from a 25:1 ratio to as much as an ultra-lean, 100:1 ratio (vs. a typical gasoline engine running 12:1 to 16:1). Diesel’s auto ignition temperature of approximately 410 degrees F (vs. 495 degrees F in gasoline) also means that the initial flame front during combustion is slightly cooler.

 

Controlling Coolant, Oil, Air, And Exhaust Temperature

Make no mistake, diesel engines are intended to perform heavy duty jobs—and when they do they definitely heat up. But to ensure they see safe coolant, oil, air intake and exhaust gas temperatures when loaded, most diesels benefit from being equipped with an oversize radiator, a large oil cooler, exceptional oil capacity, and intercoolers (be they air-to-air or air-to-water). With all of the above components working in perfect harmony, diesel engines remain in their peak efficiency range when performing tough tasks—hence their remarkable fuel economy and long-term reliability despite their huge workloads.

 

Durability Hindrances In The Modern Age

Any seasoned technician turning wrenches on diesel engines in the modern era will tell you it’s getting tougher to keep a diesel engine alive—at least as long as we’re used to seeing them last. The reason? Emissions equipment. Although emissions equipment has made today’s diesel engines cleaner than they’ve ever been (while simultaneously being the most powerful they’ve ever been), exhaust aftertreatment systems in particular have had a debilitating effect on longevity. Exhaust gas recirculation (EGR), the primary source for lowering NOx emissions, reintroduces exhaust gases into the intake tract in order to lower in-cylinder combustion temperatures. This in turn contaminates the engine oil (namely by way of adding soot, or ST on an oil analysis, to its chemical makeup). Contaminated engine oil makes it less effective in performing its most important duty, providing proper lubrication for the engine.

 

EGR Buildup

Over time, and through a combination of soot, carbon, and oil vapor buildup, EGR leads to restricted passageways within the engine’s air intake circuit. Partially and even fully clogged intake manifolds and intake ports, debris accumulation on intake valves and EGR valves can lead to a loss in power, excessive heat, and dismal engine performance. Buildup in the EGR valve is a notorious issue for modern diesel engines. When this occurs, the valve can no longer open and close as precisely (or fully), leading to poor engine performance, increased emissions, and a check engine light. Regular EGR system cleanings are highly suggested for any diesel engine in the 2002-current era.

 

EGR Is Hard On Coolant

On top of that, EGR must be cooled prior to reentering the intake tract, and most manufacturers use the engine’s cooling system to handle this task. This job—which calls for dropping exhaust gas temperatures from as high as 1,200 to 1,300 degrees F to 300 or 400 degrees F before being routed back into the intake—adds one more duty to the engine coolant’s list of things to remove heat from. In every case, EGR is hard on engine coolant, ultimately shaving lots of life off of its service interval. At the very least, antifreeze tests should be scheduled and performed regularly to ensure the engine’s coolant is up to snuff.

 

DPF Regeneration

While diesel particulate filters have their own flaws and failure points, they don’t necessarily effect a modern diesel engine’s longevity. However, DPF systems void of an additional fuel injector downstream in the exhaust aftertreatment system are prone to washing down the engine’s cylinders, overtime. Although using one or two of the engine’s fuel injectors to initiate the DOC light-off in order to carry out DPF regeneration intervals (i.e. cleaning the DPF) is less complex, the excess fuel that’s allowed to leave during the exhaust stroke washes down cylinder walls and also contaminates the engine oil. This is where the term “making oil” stems from on modern, DPF-equipped diesel engines.

 

Proper Maintenance Is The Last Piece Of The Million-Mile Puzzle

Regular and proper maintenance is crucial to keeping any diesel engine alive for the long haul. First and foremost, follow the engine manufacturer’s recommended oil change intervals. Remember, oil not only lubricates an engine, but it also serves to cool and clean it. Keeping clean engine oil in the crankcase is paramount. In some applications, severe duty service schedules should be adhered to, which can dictate that oil, fuel, and air filter changes be carried out ahead of the manufacturer’s normal intervals, along with coolant and transmission flushes. It’s also important to keep in mind that pouring quality, clean diesel fuel into the tank is part of a well-rounded maintenance regimen. Few things can cripple a diesel engine quicker than running contaminated fuel through the injection system.

 

Don’t Neglect Emissions System Maintenance

Modern emissions-curbing systems such as EGR, DPF, and SCR aren’t going anywhere—and there are a host of components within these complex systems that need to be in good working order at all times. If your diesel engine is furnished with EGR or any exhaust aftertreatment system, regular cleanings can help mitigate or altogether avoid many of the problems associated with these systems. Frequent manual regenerations (cleanings) can help keep the DPF’s ash accumulation level low, taking care that no debris makes it into the diesel exhaust fluid (DEF) tank, inspecting emissions components during regular servicing procedures, and even keeping pattern problem spare parts on-hand can save you a lot of headache as the miles and hours rack up.

 

The Million-Mile Club—Isn’t Hard To Get Into

As it pertains to “racking up miles,” no other internal combustion engine on the planet has the mechanical makeup to last as long as a diesel. In the world of Owner-Operator Class 8 trucks, seeing a million miles on the odometer is in no way an anomaly. Any engine that’s overbuilt at the factory and that is treated to proper maintenance practices throughout the course of its life can end up here—and in fact tens of thousands of engines do. The diesel powerhouse shown here is a Caterpillar 3406, an engine that has long-enjoyed a reputation in the trucking industry for being as durable as it is powerful. This yellow beast had yet to be overhauled as it crossed over the 1,000,000-mile mark.

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