3.3 Gasoline And Diesel Industrial Engines

3.3.1 General

The engine category addressed by this section covers a wide variety of industrial applications

of both gasoline and diesel internal combustion (IC) engines such as aerial lifts, fork lifts, mobile

refrigeration units, generators, pumps, industrial sweepers/scrubbers, material handling equipment (such

as conveyors), and portable well-drilling equipment. The three primary fuels for reciprocating IC

engines are gasoline, diesel fuel oil (No.2), and natural gas. Gasoline is used primarily for mobile and

portable engines. Diesel fuel oil is the most versatile fuel and is used in IC engines of all sizes. The

rated power of these engines covers a rather substantial range, up to 250 horsepower (hp) for gasoline

engines and up to 600 hp for diesel engines. (Diesel engines greater than 600 hp are covered in

Section 3.4, "Large Stationary Diesel And All Stationary Dual-fuel Engines".) Understandably,

substantial differences in engine duty cycles exist. It was necessary, therefore, to make reasonable

assumptions concerning usage in order to formulate some of the emission factors.

3.3.2 Process Description

All reciprocating IC engines operate by the same basic process. A combustible mixture is first

compressed in a small volume between the head of a piston and its surrounding cylinder. The mixture

is then ignited, and the resulting high-pressure products of combustion push the piston through the

cylinder. This movement is converted from linear to rotary motion by a crankshaft. The piston

returns, pushing out exhaust gases, and the cycle is repeated.

There are 2 methods used for stationary reciprocating IC engines: compression ignition (CI)

and spark ignition (SI). This section deals with both types of reciprocating IC engines. All dieselfueled engines are compression ignited, and all gasoline-fueled engines are spark ignited.

In CI engines, combustion air is first compression heated in the cylinder, and diesel fuel oil is

then injected into the hot air. Ignition is spontaneous because the air temperature is above the

autoignition temperature of the fuel. SI engines initiate combustion by the spark of an electrical

discharge. Usually the fuel is mixed with the air in a carburetor (for gasoline) or at the intake valve

(for natural gas), but occasionally the fuel is injected into the compressed air in the cylinder.

CI engines usually operate at a higher compression ratio (ratio of cylinder volume when the

piston is at the bottom of its stroke to the volume when it is at the top) than SI engines because fuel is

not present during compression; hence there is no danger of premature autoignition. Since engine

thermal efficiency rises with increasing pressure ratio (and pressure ratio varies directly with

compression ratio), CI engines are more efficient than SI engines. This increased efficiency is gained

at the expense of poorer response to load changes and a heavier structure to withstand the higher

pressures.1

3.3.3 Emissions

Most of the pollutants from IC engines are emitted through the exhaust. However, some total

organic compounds (TOC) escape from the crankcase as a result of blowby (gases that are vented from

the oil pan after they have escaped from the cylinder past the piston rings) and from the fuel tank and

carburetor because of evaporation. Nearly all of the TOCs from diesel CI engines enter the

10/96 Stationary Internal Combustion Sources 3.3-1
atmosphere from the exhaust. Evaporative losses are insignificant in diesel engines due to the low

volatility of diesel fuels.

The primary pollutants from internal combustion engines are oxides of nitrogen (NOx), total

organic compounds (TOC), carbon monoxide (CO), and particulates, which include both visible

(smoke) and nonvisible emissions. Nitrogen oxide formation is directly related to high pressures and

temperatures during the combustion process and to the nitrogen content, if any, of the fuel. The other

pollutants, HC, CO, and smoke, are primarily the result of incomplete combustion. Ash and metallic

additives in the fuel also contribute to the particulate content of the exhaust. Sulfur oxides (SOx) also

appear in the exhaust from IC engines. The sulfur compounds, mainly sulfur dioxide (SO2), are

directly related to the sulfur content of the fuel.2

3.3.3.1 Nitrogen Oxides -

Nitrogen oxide formation occurs by two fundamentally different mechanisms. The

predominant mechanism with internal combustion engines is thermal NOx which arises from the

thermal dissociation and subsequent reaction of nitrogen (N2) and oxygen (O2) molecules in the

combustion air. Most thermal NOx is formed in the high-temperature region of the flame from

dissociated molecular nitrogen in the combustion air. Some NOx, called prompt NOx, is formed in the

early part of the flame from reaction of nitrogen intermediary species, and HC radicals in the flame.

The second mechanism, fuel NOx, stems from the evolution and reaction of fuel-bound nitrogen

compounds with oxygen. Gasoline, and most distillate oils have no chemically-bound fuel N2 and

essentially all NOx formed is thermal NOx.

3.3.3.2 Total Organic Compounds -

The pollutants commonly classified as hydrocarbons are composed of a wide variety of organic

compounds and are discharged into the atmosphere when some of the fuel remains unburned or is only

partially burned during the combustion process. Most unburned hydrocarbon emissions result from

fuel droplets that were transported or injected into the quench layer during combustion. This is the

region immediately adjacent to the combustion chamber surfaces, where heat transfer outward through

the cylinder walls causes the mixture temperatures to be too low to support combustion.

Partially burned hydrocarbons can occur because of poor air and fuel homogeneity due to

incomplete mixing, before or during combustion; incorrect air/fuel ratios in the cylinder during

combustion due to maladjustment of the engine fuel system; excessively large fuel droplets (diesel

engines); and low cylinder temperature due to excessive cooling (quenching) through the walls or early

cooling of the gases by expansion of the combustion volume caused by piston motion before

combustion is completed.2

3.3.3.3 Carbon Monoxide -

Carbon monoxide is a colorless, odorless, relatively inert gas formed as an intermediate

combustion product that appears in the exhaust when the reaction of CO to CO2 cannot proceed to

completion. This situation occurs if there is a lack of available oxygen near the hydrocarbon (fuel)

molecule during combustion, if the gas temperature is too low, or if the residence time in the cylinder

is too short. The oxidation rate of CO is limited by reaction kinetics and, as a consequence, can be

accelerated only to a certain extent by improvements in air and fuel mixing during the combustion

process.2-3

3.3-2 EMISSION FACTORS 10/96
3.3.3.4 Smoke and Particulate Matter -

White, blue, and black smoke may be emitted from IC engines. Liquid particulates appear as

white smoke in the exhaust during an engine cold start, idling, or low load operation. These are

formed in the quench layer adjacent to the cylinder walls, where the temperature is not high enough to

ignite the fuel. Blue smoke is emitted when lubricating oil leaks, often past worn piston rings, into the

combustion chamber and is partially burned. Proper maintenance is the most effective method of

preventing blue smoke emissions from all types of IC engines. The primary constituent of black

smoke is agglomerated carbon particles (soot) formed in regions of the combustion mixtures that are

oxygen deficient.2

3.3.3.5 Sulfur Oxides -

Sulfur oxides emissions are a function of only the sulfur content in the fuel rather than any

combustion variables. In fact, during the combustion process, essentially all the sulfur in the fuel is

oxidized to SO2. The oxidation of SO2 gives sulfur trioxide (SO3), which reacts with water to give

sulfuric acid (H2SO4), a contributor to acid precipitation. Sulfuric acid reacts with basic substances to

give sulfates, which are fine particulates that contribute to PM-10 and visibility reduction. Sulfur

oxide emissions also contribute to corrosion of the engine parts.2-3

3.3.4 Control Technologies

Control measures to date are primarily directed at limiting NOx and CO emissions since they

are the primary pollutants from these engines. From a NOx control viewpoint, the most important

distinction between different engine models and types of reciprocating engines is whether they are

rich-burn or lean-burn. Rich-burn engines have an air-to-fuel ratio operating range that is near

stoichiometric or fuel-rich of stoichiometric and as a result the exhaust gas has little or no excess

oxygen. A lean-burn engine has an air-to-fuel operating range that is fuel-lean of stoichiometric;

therefore, the exhaust from these engines is characterized by medium to high levels of O2. The most

common NOx control technique for diesel and dual-fuel engines focuses on modifying the combustion

process. However, selective catalytic reduction (SCR) and nonselective catalytic reduction (NSCR)

which are post-combustion techniques are becoming available. Controls for CO have been partly

adapted from mobile sources.4

Combustion modifications include injection timing retard (ITR), preignition chamber

combustion (PCC), air-to-fuel ratio adjustments, and derating. Injection of fuel into the cylinder of a

CI engine initiates the combustion process. Retarding the timing of the diesel fuel injection causes the

combustion process to occur later in the power stroke when the piston is in the downward motion and

combustion chamber volume is increasing. By increasing the volume, the combustion temperature and

pressure are lowered, thereby lowering NOx formation. ITR reduces NOx from all diesel engines;

however, the effectiveness is specific to each engine model. The amount of NOx reduction with ITR

diminishes with increasing levels of retard.4

Improved swirl patterns promote thorough air and fuel mixing and may include a

precombustion chamber (PCC). A PCC is an antechamber that ignites a fuel-rich mixture that

propagates to the main combustion chamber. The high exit velocity from the PCC results in improved

mixing and complete combustion of the lean air/fuel mixture which lowers combustion temperature,

thereby reducing NOx emissions.4

10/96 Stationary Internal Combustion Sources 3.3-3
The air-to-fuel ratio for each cylinder can be adjusted by controlling the amount of fuel that

enters each cylinder. At air-to-fuel ratios less than stoichiometric (fuel-rich), combustion occurs under

conditions of insufficient oxygen which causes NOx to decrease because of lower oxygen and lower

temperatures. Derating involves restricting the engine operation to lower than normal levels of power

production for the given application. Derating reduces cylinder pressures and temperatures, thereby

lowering NOx formation rates.4

SCR is an add-on NOx control placed in the exhaust stream following the engine and involves

injecting ammonia (NH3) into the flue gas. The NH3 reacts with NOx in the presence of a catalyst to

form water and nitrogen. The effectiveness of SCR depends on fuel quality and engine duty cycle

(load fluctuations). Contaminants in the fuel may poison or mask the catalyst surface causing a

reduction or termination in catalyst activity. Load fluctuations can cause variations in exhaust

temperature and NOx concentration which can create problems with the effectiveness of the SCR

system.4

NSCR is often referred to as a three-way conversion catalyst system because the catalyst

reactor simultaneously reduces NOx, CO, and HC and involves placing a catalyst in the exhaust stream

of the engine. The reaction requires that the O2 levels be kept low and that the engine be operated at

fuel-rich air-to-fuel ratios.4

The most accurate method for calculating such emissions is on the basis of "brake-specific"

emission factors (pounds per horsepower-hour [lb/hp-hr]). Emissions are the product of the brakespecific emission factor, the usage in hours, the rated power available, and the load factor (the power

actually used divided by the power available). However, for emission inventory purposes, it is often

easier to assess this activity on the basis of fuel used.

Once reasonable usage and duty cycles for this category were ascertained, emission values

were aggregated to arrive at the factors for criteria and organic pollutants presented. Factors in

Table 3.3-1 are in pounds per million British thermal unit (lb/MMBtu). Emission data for a specific

design type were weighted according to estimated material share for industrial engines. The emission

factors in these tables, because of their aggregate nature, are most appropriately applied to a population

of industrial engines rather than to an individual power plant. Table 3.3-2 shows unweighted speciated

organic compound and air toxic emission factors based upon only 2 engines. Their inclusion in this

section is intended for rough order-of-magnitude estimates only.

Table 3.3-3 summarizes whether the various diesel emission reduction technologies (some of

which may be applicable to gasoline engines) will generally increase or decrease the selected

parameter. These technologies are categorized into fuel modifications, engine modifications, and

exhaust after-treatments. Current data are insufficient to quantify the results of the modifications.

Table 3.3-3 provides general information on the trends of changes on selected parameters.

3.3-4 EMISSION FACTORS 10/96
3.3.5 Updates Since the Fifth Edition

The Fifth Edition was released in January 1995. Revisions to this section since that date are

summarized below. For further detail, consult the memoranda describing each supplement or the

background report for this section.

Supplement A, February 1996

No changes.

Supplement B, October 1996

Text was revised concerning emissions and controls.

The CO2 emission factor was adjusted to reflect 98.5 percent conversion efficiency.

10/96 Stationary Internal Combustion Sources 3.3-5
3.3-6 EMISSION FACTORS 10/96

Table 3.3-1. EMISSION FACTORS FOR UNCONTROLLED GASOLINE

AND DIESEL INDUSTRIAL ENGINESa

Pollutant

Gasoline Fuel

(SCC 2-02-003-01, 2-03-003-01)

Diesel Fuel

(SCC 2-02-001-02, 2-03-001-01)

EMISSION

FACTOR

RATING

Emission Factor

(lb/hp-hr)

(power output)

Emission Factor

(lb/MMBtu)

(fuel input)

Emission Factor

(lb/hp-hr)

(power output)

Emission Factor

(lb/MMBtu)

(fuel input)

NOx 0.011 1.63 0.031 4.41 D

CO 6.96 E-03d 0.99d 6.68 E-03 0.95 D

SOx 5.91 E-04 0.084 2.05 E-03 0.29 D

PM-10b 7.21 E-04 0.10 2.20 E-03 0.31 D

CO2

c 1.08 154 1.15 164 B

Aldehydes 4.85 E-04 0.07 4.63 E-04 0.07 D

TOC

 Exhaust 0.015 2.10 2.47 E-03 0.35 D

 Evaporative 6.61 E-04 0.09 0.00 0.00 E

 Crankcase 4.85 E-03 0.69 4.41 E-05 0.01 E

 Refueling 1.08 E-03 0.15 0.00 0.00 E

a References 2,5-6,9-14. When necessary, an average brake-specific fuel consumption (BSFC) of

7,000 Btu/hp-hr was used to convert from lb/MMBtu to lb/hp-hr. To convert from lb/hp-hr to kg/kwhr, multiply by 0.608. To convert from lb/MMBtu to ng/J, multiply by 430. SCC = Source

Classification Code. TOC = total organic compounds. b PM-10 = particulate matter less than or equal to 10 :m aerodynamic diameter. All particulate is

assumed to be # 1 :m in size. c Assumes 99% conversion of carbon in fuel to CO2 with 87 weight % carbon in diesel, 86 weight %

carbon in gasoline, average BSFC of 7,000 Btu/hp-hr, diesel heating value of 19,300 Btu/lb, and

gasoline heating value of 20,300 Btu/lb. d Instead of 0.439 lb/hp-hr (power output) and 62.7 lb/mmBtu (fuel input), the correct emissions

factors values are 6.96 E-03 lb/hp-hr (power output) and 0.99 lb/mmBtu (fuel input), respectively.

This is an editorial correction. March 24, 2009
Table 3.3-2. SPECIATED ORGANIC COMPOUND EMISSION

FACTORS FOR UNCONTROLLED DIESEL ENGINESa

EMISSION FACTOR RATING: E

Pollutant

Emission Factor

(Fuel Input)

(lb/MMBtu)

Benzeneb 9.33 E-04

Tolueneb 4.09 E-04

Xylenesb 2.85 E-04

Propylene 2.58 E-03

1,3-Butadieneb,c <3.91 E-05

Formaldehydeb 1.18 E-03

Acetaldehydeb 7.67 E-04

Acroleinb <9.25 E-05

Polycyclic aromatic hydrocarbons (PAH)

Naphthaleneb 8.48 E-05

Acenaphthylene <5.06 E-06

Acenaphthene <1.42 E-06

Fluorene 2.92 E-05

Phenanthrene 2.94 E-05

Anthracene 1.87 E-06

Fluoranthene 7.61 E-06

Pyrene 4.78 E-06

Benzo(a)anthracene 1.68 E-06

Chrysene 3.53 E-07

Benzo(b)fluoranthene <9.91 E-08

Benzo(k)fluoranthene <1.55 E-07

Benzo(a)pyrene <1.88 E-07

Indeno(1,2,3-cd)pyrene <3.75 E-07

Dibenz(a,h)anthracene <5.83 E-07

Benzo(g,h,l)perylene <4.89 E-07

TOTAL PAH 1.68 E-04

a Based on the uncontrolled levels of 2 diesel engines from References 6-7. Source Classification

Codes 2-02-001-02, 2-03-001-01. To convert from lb/MMBtu to ng/J, multiply by 430. b Hazardous air pollutant listed in the Clean Air Act. c Based on data from 1 engine.

10/96 Stationary Internal Combustion Sources 3.3-7
Table 3.3-3. EFFECT OF VARIOUS EMISSION CONTROL TECHNOLOGIES

ON DIESEL ENGINESa

Technology

Affected Parameter

Increase Decrease

Fuel modifications

Sulfur content increase PM, wear

Aromatic content increase PM, NOx

Cetane number PM, NOx

10% and 90% boiling point PM

Fuel additives PM, NOx

Water/Fuel emulsions NOx

Engine modifications

Injection timing retard PM, BSFC NOx, power

Fuel injection pressure PM, NOx

Injection rate control NOx, PM

Rapid spill nozzles PM

Electronic timing & metering NOx, PM

Injector nozzle geometry PM

Combustion chamber modifications NOx, PM

Turbocharging PM, power NOx

Charge cooling NOx

Exhaust gas recirculation PM, power, wear NOx

Oil consumption control PM, wear

Exhaust after-treatment

Particulate traps PM

Selective catalytic reduction NOx

Oxidation catalysts TOC, CO, PM

a Reference 8. PM = particulate matter. BSFC = brake-specific fuel consumption.

3.3-8 EMISSION FACTORS 10/96
References For Section 3.3

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10/96 Stationary Internal Combustion Sources 3.3-9