Let me start with a bit of a disclaimer. I drive a diesel pickup. It’s a phenomenon that can only be understood by members of certain groups. Similar to riding a motorcycle or driving a Jeep Wrangler (both of which I have done in the past), where everyone who is doing likewise waves as they pass, owning a diesel pickup truck is a bit of a loosely assembled but wildly loyal brotherhood. So it is with a bit of a prejudice, and a pair of work boots that reek of spilled diesel and motor oil that I write this post.
To understand the question, it is best that we start by understanding the differences between the two designs. Both types of engines are internal combustion, reciprocating piston designs. For purposes of this discussion, I will be referring to 4 stroke conventional engines, not rotary, 2 stroke, or turbine engines.
Let’s start by talking about the two fuels for a minute. Gasoline is a more refined fuel (meaning more impurities have to be removed from crude oil to make gasoline). Diesel is less refined. Diesel makes a better lubricant and burns slowly. Gasoline is an excellent solvent and burns rapidly (almost explodes). An interesting sidebar here is that since diesel fuel is less refined than gasoline, you get more gallons of diesel out of a barrel of crude oil than you do gasoline. This begs the question then “why is diesel more expensive than gasoline?” doesn’t it? The answer is a combination of market forces, but a large portion of the difference lies in the taxes imposed upon each type of fuel. Diesel is perceived at this point to be a fuel “for industry” and thus Uncle Sam assumes they can bear a larger “donation” to the federal coffers.
The main difference in the way that these two engine designs work is their method if ignition. A gas engine compresses a mixture of air and fuel (gasoline) to a relatively low compression ratio (typically around 8-12:1) and then ignites the mixture with a spark by means of a spark plug. A diesel engine on the other hand compresses the air and fuel until the mixture explodes by spontaneous combustion because of the pressures to which it has been compressed. This is why a diesel engine is sometimes referred to as a compression ignition (or combustion) engine. It compresses the air/fuel mixture to the point of combustion. As a result of this difference, the diesel engine tends to run at a much higher compression ratio than its gas counterpart (in the range of 14:1 to 25:1).
Also noteworthy is the difference in the fuel delivery to the combustion chamber. A gasoline engine always has a butterfly valve (air to fuel ratio is critical to the operation of a gasoline engine) in the air intake system to restrict the flow of air to maintain a very delicate balance of air and fuel. The fuel is delivered by means of either a carburetor or is injected at relatively low pressures through a fuel injection nozzle into the intake tack of the engine. A few manufacturers have started using DI (Direct Injection) technology as well in their gasoline engines, but they are the exception and not the rule at this point. A diesel engine generally has no restriction in the air intake to restrict the flow of air so it runs inherently lean (meaning that it has a lot more air than it does fuel in the mix). This also means that the diesel engine has less resistance when pulling the air into a cylinder, and flows a larger volume of air at partial throttle application. Essentially a diesel engine uses about the same volume of air per revolution at idle as a gasoline engine does at full throttle. A diesel engine generally injects the fuel into the combustion cylinder either by way of a mixing chamber (IDI or InDirect Injection) or sprays the fuel right onto the top of the piston (DI or Direct Injection).
Okay so all the techno babble aside, what does all that jargon mean?
Lemme’ break it down for ‘ya, brotha:
–In general a diesel engine runs slower than a gasoline engine. This is not always true but is a general rule. Where as most automotive engines redline (have their speeds limited) at around 5,000 RPM’s the 7.3 liter turbo diesel in my truck redlines at 3,200 RPM’s. This is not a huge difference, but it does have an affect on the way the vehicle drives.
–Diesel engines tend to be more durable. Their fuel is a lubricant fuel. Diesel is “slippery” compared to gasoline, so it tends to help lubricate the engine as it is being burned. Add to that the fact that the engines have to be built heavier to withstand the combustion forces, and the result is generally a more durable design.
–Diesels can run on alternative fuels. Rudolf Diesel (whose engine design bears his name) actually pioneered the technology to run off of powdered coal dust, and then later adapted it to run off liquid oils. There are plenty of online resources devoted to the use of Waste Vegetable Oil (WVO), Waste Motor Oil (WMO), or home brewed biodiesel as motor fuels.
–Weight is a factor. The fact that a diesel engine runs with higher compression ratios means that it has to be built heavier in order to withstand the higher pressures inside it. Don’t look for a diesel powered chainsaw anytime soon. The engine that is in my truck weighs about a half a ton (literally). That is about half again what a typical gas engine of similar size would weigh.
–Typically a diesel engine produces more torque than an equally sized gasoline engine. This is splitting hairs to some degree since horse power is the ultimate measure of an engine’s ability to do work, but since horsepower is a function of speed and torque (horsepower = torque times speed, divided by 5252 or HP=(TQ*RPM)/5252 in algebraic terms) driving a diesel just feels different.Torque is the measure of force, but horsepower takes into account how fast you can apply that force.
–Fuel consumption is generally less with a diesel engine. This is because of the fact that there are fewer mechanical losses in the diesel system (remember that restricting throttle butterfly we talked about up a few paragraphs?) and the fact that the diesel is running at a higher compression ratio.
Essentially here is the real world application.My previous truck was a 5.4 liter V-8 gasoline powered truck. The engine was rated at 260 HP and 290 lb/ft of torque. It’s redline was around 5,000 RPM. This truck weighed around 5,600 pounds would seat two adults and three people who didn’t mind being sandwiched into the back jump-seat. It averaged about 14 MPG of mixed driving and had a carrying capacity of about 1,200 lbs. My current truck (I can’t say “new since it is 3 years older than my “old” one) uses a 7.3 liter V-8 turbo diesel power plant which is rated at 215 HP and generates 450 lb/ft of torque. This truck weighs about 6,700 lbs, seats 6 full sized people and can haul about twice as much in the bed as the prior ride could. And by the way I have averaged 14.75 MPG of mixed driving as of late. This pretty well describes the difference in these two designs. Gasoline engines make good power but you generally have to wind them up to get the most out of them. Diesel engines aren’t going to rev as far as a gasoline engine, but their higher compression ratios make them very strong (meaning a lot of torque) at lower speeds.
So which one is better? Well the answer is -BOTH. No, seriously; it really depends on the application. As I have said I am a fan of diesels because of their more efficient design, their durability and their fuel efficiency, but there are some factors that just make them the wrong tool for the job in some instances.
–While a diesel truck may carry more firewood and use the same or less fuel, it isn’t well suited for short trips. That 1,000 lbs of cast iron under the hood takes forever to warm up in the Ohio winter so don’t get in a hurry to thaw your toes or defrost the windshield unless you had the block heater plugged in. Block heater, that reminds me of another downfall of a diesel engine.
–They don’t start well when cold. Much of this issue can be overcome by having a good working glow plug system that essentially heats each cylinder individually when the engine is started, but they certainly don’t like the cold.
–Weight is also an issue. The diesel design just requires a heavier engine. This makes them poorly suited to applications where weight is a determining factor. While weight may not be a huge issue if you’re driving a full size pickup truck, locomotive, tractor trailer, farm tractor, or piece of heavy machinery, for applications where weight is a consideration like portable power equipment, aircraft, etc the weight of a diesel power plant is a major limitation.
–Maintenance on diesels is more intensive. Number one there is typically a lot more motor oil in a diesel engine (there are close to 4 gallons of oil in my truck right now as compared to 5 quarts in most automobile gasoline engines). Every oil change I perform costs me about $60. And that is doing it myself (of course). Diesels also have to have special systems that a gasoline engine doesn’t (turbo chargers, vacuum pumps, glow plug systems, oil coolers, high pressure oil or fuel systems, etc).
–Fuel quality is a lot more critical in a diesel engine, especially newer ones. Gasoline can absorb some water and still do its job just fine. Diesel and water will not mix under general circumstances. It has been a disturbing but quiet story for a while now that many of the newer automotive diesel engines have been very, VERY sensitive to fuel quality issues. I heard a story the other day where a gentleman was on the hook with his dealership for about a $10,000 repair bill for his new diesel truck because the fuel had water in it.
–Emissions standards are harder to achieve with a diesel engine. I’m sure we all have seen a diesel truck or two go down the road billowing black smoke out the stacks. It takes a lot of technology working together to keep the engine making peak power without blowing smoke like that. There are some newer diesels that actually require additional urea injection systems to “clean” the exhaust up and prevent that smoke. These vehicles have a separate tank for Diesel Exhaust Fluid (DEF for short). Mixing that fluid with the fuel can also ruin the fuel system.
Having a diesel ends up to a bit of a hobby for most people. Newer designs are improving this, but there are still some extra things to consider and maintain when you have a diesel engine.
The bottom line is that both designs have their strong suits and their downfalls. For a home generator, my choice is a diesel, but for a lawn mower it’s gasoline all the way. The grocery getter….. well, that one is still up for debate.