Friday, May 23, 2014
Compress This! A Diesel primer.
Ah, the poor, abused, maligned Diesel engine. Whether for the perception of being impossible to start and causing winter frustrations, or for the smokey, clattering, rattling, growling machines being perceived as being adequate only for big truck, trains and ships, diesels have somehow earned a bad name. All this is nonsense you must realize. Diesels make fine automobile and smaller truck and equipment powerplants that can attain amazing fuel economy numbers. Diesels can be smooth, quiet, and fun to drive since by design, they have very driver- friendly torque output.
Diesels come in as wide a variety as gasoline engines or anything else for that matter, from utter and total crap to fine works of engineering mastery that will last millions of miles. From single cylinder consumer powerheads for generators and mowers, to massive ship engines that are the size of a large house.
Most of the world outside the US is powered by diesel engines. Why the allergy in the US? I blame GM and Ford. There, I said it. I do. Back in the late 70's and early 80's, GM made some of the worst diesels ever for the consumer market. By contrast, GM's Detroit Diesel series engines are some of the finest engines ever produced. Period. So GM cannot claim ignorance. I could go on and on about GM and its 75 year list of royal screw-ups.
Call me conspiratorial, but I say they planned and arranged it to cause American consumers to grow wary and afraid of diesels so that GM could benefit from more sales of cheap gasoline engine which provide much higher profit margins than better built diesel engines. Ford seems to have had almost the same plan when they ruined a perfectly good International engine design. Sorry guys, but until Cummins and Dodge changed things around the US, the diesel engine was a marginalized mongrel red headed step child in the consumer market.
Sheesh... I digress. Enough tomfoolery... back to business. The differences between gasoline fueled spark-ignition (SI), and oil fueled, compression ignition (CI) Diesel engines are multifold, and not only physical, but also chemical in nature- here is the part you came here for.
Since the mixture must always remain stoichiometric in a SI engine, a throttle butterfly valve is used to control the amount of overall mix going into each cylinder, thus controlling the output power and speed. Throttle valves makes the engine work harder to get the mixture they need to run- that is, an SI engine must waste energy just to get its energy source in!!
The term is 'volumetric efficiency': it refers to the ratio of how much 'stuff' fills a space vs. how much stuff that space COULD hold. The lower the ratio of gasses:space inside the cylinder -the emptier the cylinder is- the less output power, and the more waste in 'sucking' air against that closed throttle. With diesels, the amount of air is unregulated, thus the cylinders always get a full load of air to compress with tiny amounts of fuel injected to make power. Most diesels run at very high (nearly 100%) volumetric efficiencies. SI engines only approach these high volumetric efficiencies under wide-open-throttle (WOT) conditions at the tuned maximum torque point of the engines RPM band.
Diesel fuel itself also has inherently more chemical energy (more C=C bonds) than gasoline as well, allowing a diesel engine to use less fuel for the same energy output. Using less fuel to do the same work is one of the hallmarks of a diesel engine. A distributor in a gasoline engine is used to transfer the spark to the correct cylinder at the correct instant (now-days its computer controlled), but in diesels its a measured amount of fuel that is distributed to the correct cylinder at the correct time (again, in recent years, controlled by computer). No spark is needed in CI engines because the temperature of the air after compression is high enough to spontaneously ignite the fuel.
Work done in compression is known by the term 'adiabatic' which means that the work done on the upstroke in compressing those gasses is returned during expansion on the next downstroke. No energy- or at least very little- is lost due to adiabatic compression in an already hot engine. Diesels are built to tighter tolerances and of (usually) higher quality materials to create a higher compression ratio and combustion pressures inside the cylinders, thus higher temperatures sufficient for self ignition of the fuel and faster flame propagation. The result of this higher temperature and pressure compression stroke is the more complete burn of the fuel creating higher efficiency overall.
Being built out of better materials and of heavier construction to tolerate higher compression and combustion pressures also means that diesel engines will outlast most gasoline engines by many times the lifespan. The stronger and more rigid engine block and head design means that the engine is highly unlikely to twist, bend, or distort under high stress. Diesels in full size highway trucks are designed, built, and maintained to go 1 or 2 million miles (or more) between rebuilds, smaller designs can last nearly as impressively though too.
The energy released by combustion and expansion of burnt gasses is introduced immediately after the time of fuel injection. The more fuel that is injected (to a point), the greater the amount of created gasses and heat. Expansion pressure and power output is directly proportional to the amount of fuel injected. The ability to inject fuel in a finely atomized spray against high cylinder pressure is made possible by specially built pumps that can make many thousands of PSI to create effective spray and burn patterns. To prevent excessive soot and other emissions as well as maximizing power, the finest spray possible is used- the finer the spray, the faster and more complete the burn.
On newer diesel designs, the fuel is forced to spray out through holes in the nozzle of the injector that are too small to be seen with the naked eye. A high-pressure fuel injection pump that is capable of generating the many thousands of PSI needed to force the fuel out of these tiny atomization ports need alot of power to do its job. This high pressure pump can consume a large amount of power from the engine that cannot go to the wheels- thus the payoffs may not outweigh the energy investment to get the cleaner burn that comes with finer spray patterns. This higher pumping cost is one of the reasons that newer diesels don't get better fuel economy- that and all the emissions controls choking them to death (I smell another rant coming on...)
So because diesels are built better, have higher compression ratios and more complete burn of fuel, do not suffer pumping losses caused by a throttle plate, burn higher energy- density fuel, and are much simpler due to lack of electrical ignition system, diesels last longer and operate at much higher efficiencies than their spark-ignition cousins. Give fair consideration to diesel powered machines next time you are pondering a new piece of equipment.
Hope that clarified a few things regarding diesel engines. “All hail Rudolph Diesel!” Til next time! MW...