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Gas vs Diesel

The Advantages and Disadvantages of Gasoline and Diesel Engines

The debate between gasoline and diesel engines has become somewhat quintessential with the substantial growth of available diesel powered vehicles in the United States. Elsewhere in the world, diesel engines have reined supreme as a result of their fuel efficiency and longevity. However, there are many factors to consider when selecting an engine type and diesel isn't necessarily a cost effective solution to every driver's needs.

Engine Efficiency

Diesel engines are capable of much higher fuel economy than a gasoline engine for many reasons. It is not uncommon for a diesel engine to consume 10% to 30% less fuel than a comparable gasoline engine, and fuel efficiency does not taper off as rapidly in diesel engines under high load driving scenarios.

High Thermal Efficiency

A diesel engine's most profound advantage is, arguably, its relatively high thermal efficiency. The thermal efficiency of a process indicates how efficiently the engine can convert heat energy from combustion into usable mechanical energy. A major contributing factor, if not the most important characteristic of a diesel engine is its relatively high compression ratio.

While compression ratios vary between engine designs (indirect diesel engines typically utilize higher compression ratios than direct injection engines, for example) a diesel engine will typically employ a compression ratio in the 16:1 to 22:1 range. On the contrary, a typical gasoline engine will exhibit a compression ratio in the 9:1 to 12:1 range do to the fact that a gasoline engine is susceptible to detonation (fuel igniting prematurely during the compression stroke). Detonation is not a concern in diesel engines because they do not compress air and fuel as a mixture - fuel is not injected into the engine until ignition is desired, thus high compression ratios are both practical and attainable in diesel engines.

Higher compression ratios result in a higher rate of expansion of gases during the power stroke, which converts a greater amount of heat energy into mechanical energy than a comparable engine with a lower compression ratio. The result is relatively cool exhaust gas temperatures, translating into a higher thermal efficiency since more of the heat created during combustion has been consumed in moving the piston downwards.

Energy Content of Diesel Fuel vs Gasoline

Diesel fuel contains on the order of 10% more potential energy per volumetric unit than gasoline. The following table of values has been provided by the United States Department of Energy Alternative Fuels Data Center:

 

Gasoline (E10)

Ethanol (E100)

Diesel Fuel (ULSD)

B100 Biodiesel

Lower Heating Value

112,000 - 116,000 BTU/gal

76,000 BTU/gal

128,500 BTU/gal

119,500 BTU/gal

Higher Heating Value

120,000 - 124,000 BTU/gal

84,500 BTU/gal

138,500 BTU/gal

128,000 BTU/gal

Note that "lower" and "higher" heating values are measurements of energy content with slightly different test parameters. In any case, diesel fuel has a significantly higher energy content than gasoline; per these values, a gallon of diesel fuel contains 113% of the energy contained in a gallon of gasoline (13% greater). This theoretically translates into greater fuel economy since each gallon of diesel fuel has the ability to produce more usable mechanical energy than a gallon of gasoline.

Air-Fuel-Ratio

The diesel combustion process is flexible in that a diesel engine can operate on a spectrum of air-to-fuel ratios and is not necessarily limited to its stoichiometric ratio like gasoline engines. A major contributing factor is that diesel engines do not have spark plugs, which would foul rapidly from soot contamination if the engine were operated at a rich air-fuel mixture. More importantly [for the purpose of efficiency], a diesel engine can operate with a relatively lean air-to-fuel ratio with no surmountable concerns. On the contrary, a gasoline engine will have a tendency to foul the spark plugs when operated excessively rich and combustion temperatures will have a tendency to reach concernable limits when operated excessively lean. The ability to run a relatively lean mixture translates into reduced fuel consumption under low load conditions, i.e. cruising on level roadways.

Waste Heat Recovery

Turbocharged diesel engines (naturally aspirated engines are long defunct for on-highway applications) are able to convert waste heat generated by the engine into usable mechanical power. Hot exhaust gases leaving the combustion chamber are routed into the turbocharger turbine housing where the heat energy of the exhaust gases is converted into mechanical energy by the thermodynamic process across the turbine wheel. The resulting recovery of otherwise waste energy contributes to engine efficiency and increased overall performance potential. This allows diesel engines of smaller displacements to exhibit the performance characteristics of a significantly larger engine without the handicaps associated with a large displacement engine.

Longevity

Diesel engines are characteristically robust, do in part to the fact that they are designed to overcome the cylinder pressures associated with high compression ratios and forced induction. They are also typically under square (long stroke, small bore), requiring a reciprocating assembly with a greater mass to counter high stresses. Diesel engines also operate at low engine speeds with redlines typically in the 3,000 to 3,500 rpm range, contributing to reduced wear over the course of the engine's usable life. These factors contribute to a diesel engines life, which is significantly greater than that of any comparable mass production gasoline engine. It is not uncommon for diesel cars and pickups to exceed 500,000 miles of driving before requiring a major overhaul. Likewise, it is not uncommon for diesel engines in heavy truck (tractor trailers) applications to exceed 1,000,000 miles before requiring a major overhaul. The downfall of the engine's strength is that diesel engines tend to be significantly heavier than gasoline engines.

Low RPM Torque

Not only does diesel fuel pack more energy than gasoline, but it combusts with completely different properties than gasoline. One of the major differences is how quickly combustion occurs and, more specifically, the rate of expansion of the exhaust gases. Gasoline combusts and expands rapidly when ignited, whereas diesel fuel is a much slower burn. One could argue that the difference equates to hitting a piston rapidly with a large force versus pushing the piston downwards slowly with a large force. Both the relatively long engine stroke length and combustion characteristics of diesel engines contribute to their generous low end torque ratings, albeit at the expense of high RPM horsepower.

Cost of Ownership

Cost of ownership is an interesting topic with regards to the corresponding differences between gasoline and diesel engines. There are many factors that must be taken in consideration, including vehicle use, purchase price, estimated usable engine life, maintenance costs, and fuel costs. In many instances diesel powered vehicles are economical alternatives to gasoline vehicles. Likewise, there are many instances where a gasoline vehicle presents a more economical option when evaluating the total cost to own and operate over the course of the vehicle's usable lifespan.

With regard to trucks, diesel engines are largely preferred for their ability to tow vessels of substantial weight and will do so much more efficiently than a comparable gasoline engine within a similar performance category. However, diesel engines are typically more expensive to maintain do to more rigorous service schedules, larger crankcase oil capacities, and in some instances additional incurred costs resulting from the complexity of modern engines. Buyers of diesel vehicles will also typically encounter higher purchase prices. Another important consideration (as previously discussed) is that diesel engines typically have a [significantly] longer lifespan than gasoline engines, primarily as a result of their robust designs.

Exhaust Emissions/Combustion Byproducts

The differences in exhaust emissions between diesel and gasoline vehicles presents an interesting discussion and, without getting too carried away on technicalities, the conclusion is that diesel and gas engine emissions are different, but one is not necessarily worse than the other. Here's a breakdown of four important emissions categories:

Carbon Monoxide (CO) Emissions

Carbon monoxide (CO) is an odorless, colorless gas that is highly toxic in nature. Carbon monoxide in the exhaust stream is converted to CO2 by means of a catalyst; the catalytic converter(s) on a gasoline engine and the diesel oxidation catalyst (DOC) on diesel engines. The combustion of gasoline produces a higher concentration of carbon monoxide than diesel engines, though there is typically no surmountable debate regarding CO emissions of the two engines since methods are in place to reduce the emittance of carbon monoxide in both engines.

Carbon Dioxide (CO2) Emissions

Carbon dioxide is the "original" Greenhouse Gas and, although it is converted to oxygen by plant life, is a major contributor to global warming. Per the United States Environmental Protection Agency (EPA), burning 1 gallon of gasoline in an engine produces 19.59 lbs of CO2 while 1 gallon of diesel fuel produces 22.44 lbs of CO2. While diesel engines produce more carbon dioxide per gallon of fuel consumed, they also consume less fuel than a comparably sized diesel engine and thus are typically granted the advantage in this emissions category.

Nitrous Oxides (NOx) Emissions

"Nitrous oxides" is an industry term used to describe nitrogen dioxide (NO2) and nitric oxide (NO), both of which are compositions of nitrogen and oxygen atoms. NOx emissions contribute to visible "smog" in the lower atmosphere and is known to cause problems associated with the respiratory system. For people with preexisting medical conditions such as bronchitis and asthma, nitrous oxides have a tendency to make symptoms worse.

Nitrous oxides are combustion products in both gasoline and diesel engines, however they are much more prominent in the combustion of diesel fuel (especially when an engine is running lean). In gasoline engines, NOx emissions are reduced using the exhaust gas recirculation system (EGR). In modern diesel engines NOx emissions are also reduced using EGR, however diesel engines further employ a selective catalytic reduction (SCR) system to convert NOx gases into nitrogen gas and water vapor. The SCR system uses a urea based exhaust fluid (DEF) to enable the reduction reaction that takes place in the SCR catalyst. As a result of this aftertreatment system, NOx emissions are notably low in diesel vehicles.

Particulate Emissions

Both diesel and gasoline engines emit certain hydrocarbons or particulates. However, the combustion of diesel fuel produces substantially more particulates (soot) than that of gasoline. Exhaust soot from diesel engines is a known carcinogen and therefore a serious health concern. Fortunately, industry regulations have cut particulate emissions of diesel engines considerably with the use of diesel particulate filters (DPF), which are up to 99% effective in capturing and processing these particles and thus particulate emissions are no longer a significant concern with on-highway diesel engines.

Diesel vs Gas | The advantages and Disadvantages of Gas/Diesel Engines

 

Gasoline Engine

Diesel Engine

Advantages

Low emissions without requiring extensive emissions control equipment/devices

High thermal efficiency (up to 30% greater fuel economy)

Lightweight and comparatively small size

Fuel exhibits high energy content per volumetric unit

Generous mid-range and high rpm horsepower/torque

Flexible air-fuel ratio

Capable of operating at high engine speeds

Long lifespan, robust engine components

Low service and repair costs

Generates high torque at low engine speeds

Attractive cost of ownership/low initial purchase cost

Lack of ignition system

Low noise, vibration, and harshness

Engines run cool (rarely overheat w/ properly maintained cooling system)

Fuel is readily available

Well suited to operate continuously at high load for long periods of time

Engine design well versed for small and large engines

High resale value

Disadvantages

 

Limited lifespan

Relatively large engine size and mass

Lower thermal efficiency

Comparatively high initial purchase cost

Detonation (knocking or "ping") becomes serious concern in high compression engines

Comparatively high service and repair costs

Lower resale value (primarily do to limited lifespan and lower desirability in certain realms)

Fuel dilution is a major concern, not well suited for stop-and-go driving or long periods of idling

Fuel volatility (greater safety concerns w/ storage and filling)

Limited max engine speed

Greater payload (not trailer weight) in truck applications do to lower engine weight

Extensive, if not somewhat burdensome emissions control system (modern engines)

Ethanol in gasoline can be detrimental to fuel systems (Federal mandates in the U.S.A. require that all on-highway gasoline be blended with ethanol)

Not all fuel stations carry diesel fuel (although it is not particularly difficult to find diesel fuel in any locality)