Identification of the Problem in Controlling the Air–Fuel Mixture Ratio (Lambda Coefficient λ) in Small Spark-Ignition Engines for Positive Pressure Ventilators

Abstract

The air–fuel ratio is a crucial parameter in internal combustion engines that affects optimal engine performance, emissions, fuel efficiency, engine durability, power, and efficiency. Positive pressure ventilators (PPVs) create specific operating conditions for drive units, characterized by a reduced ambient pressure compared to standard atmospheric pressure, which is used to control carburetor-based fuel supply systems. The impact of these conditions was investigated for four commonly used PPVs (with internal combustion engines) in fire services across the European Union (EU), using a lambda (λ), carbon dioxide (CO₂), carbon monoxide (CO), and hydrogen carbon (HC) analyser for exhaust gases. All four ventilators were found to operate with lean and very lean mixtures, with their lambda coefficients ranging from 1.6 to 2.2. The conducted tests of the CO₂, CO, and HC concentrations in the exhaust gases of all four fans show dependencies consistent with theoretical analyses of the impact of the fuel–air mixture on emissions. It can be observed that as the amount of burned air decreases, the values of CO and HC decrease, while the concentration of CO₂ increases with the increase in engine load. Such an operation can accelerate engine wear, increase the emission of harmful exhaust gases, and reduce the effective performance of the device. This condition is attributed to an inadequate design process, where drive units are typically designed to operate within atmospheric pressure conditions, as is common for these engines. However, when operating with a PPV, the fan’s rotor induces significant air movement, leading to a reduction in ambient pressure on the intake side where the engine is located, thereby disrupting its proper operation.

1. Introduction

Internal combustion engines used to power positive pressure ventilators (PPVs) belong to the category of non-road small engines, which are subject to different emission standards compared to road vehicles [1,2]. Emission regulations for non-road small engines are more lenient than those for vehicles [3,4]. Therefore, their fuel and air supply systems are less technologically advanced and, among other things, can be carburetor fuel supply systems, which in road vehicles have been replaced by fuel injection systems since the 1990s due to their better precision in controlling the fuel dose [5,6]. However, in non-road small engines, apart from low emissions and efficiency, the functionality of the device equipped with this engine is also a very important factor [7,8]. For this reason, the low weight and size of the device are crucial, which are ensured by the use of a carburetor fuel and air supply system [9]. The instrumentation of modern internal combustion engines in vehicles can double the engine’s size and significantly increase its weight, making solutions with a carburetor system acceptable [2,10].

It is essential that the process of controlling the air–fuel mixture in an internal combustion engine takes into account the lambda (λ) coefficient [11]. The λ coefficient is a crucial parameter in controlling the air–fuel mixture in combustion processes [12]. It represents the ratio of the actual air–fuel ratio to the stoichiometric air–fuel ratio [13]. Understanding and controlling the lambda coefficient is necessary to optimize combustion efficiency [14], reduce emissions [15,16], and maintain engine performance. Maintaining the proper air–fuel ratio is critical to ensuring that an engine operates efficiently [17]. A mixture that is too rich (too much fuel) or too lean (too little fuel) can lead to engine performance issues. The lambda coefficient directly affects exhaust emissions. A rich mixture can lead to increased emissions of carbon monoxide (CO) and hydrocarbons (HCs), while a lean mixture can lead to increased emissions of nitrogen oxides (NO_x). Correctly setting the λ coefficient can help optimize fuel consumption, which is beneficial both for the environment and for the vehicle owner’s budget [18]. The prolonged use of an incorrect λ coefficient can lead to the premature wear of engine components such as valves, pistons, and the catalytic converter [19,20,21]. Maintaining the appropriate λ coefficient is crucial for achieving maximum engine power and efficiency. Incorrect mixture ratios can result in power loss and poor engine responsiveness. Therefore, monitoring and adjusting the λ coefficient is a critical aspect for both engineers designing engines and mechanics ensuring their proper operation [22].

The optimal air–fuel ratio depends on various factors, such as the type of engine, the operating conditions, and the desired outcome (e.g., maximum power, fuel economy, emission minimization). Here are some general guidelines. For gasoline engines, the stoichiometric ratio is achieved with a lambda (λ) coefficient of 1 [15]. This means that 14.7 parts of air are required to burn 1 part of fuel completely, without excess oxygen or fuel. To achieve maximum engine power, a richer mixture is used, typically with a λ coefficient ranging from 0.85 to 0.90. A richer mixture provides additional cooling and allows for better performance. To achieve optimal fuel economy, the mixture is usually slightly leaner than a stoichiometric mixture, with a λ coefficient ranging from 1.05 to 1.12. This mixture is less fuel-intensive but can lead to higher exhaust temperatures. For minimizing emissions, especially nitrogen oxides (NO_x), a stoichiometric ratio with a λ coefficient of 1 is used because the three-way catalytic converter is most effective at this ratio [23]. Under different engine operating conditions, such as cold starts [8], acceleration, or driving under load, the λ coefficient can be adjusted. For example, during a cold start, a richer mixture is used to ensure stable engine operation (a λ coefficient ranging from 0.816 to 0.884). Example λ values for idling are λ = 1, while for cruising they range from 1.02 to 1.088. The optimal λ coefficient depends on the specific requirements and operating conditions of the engine. Therefore, modern engine management systems (ECUs) dynamically adjust their AFR to ensure the best performance and efficiency in various situations [24].

The application of a carburetor system for powering positive pressure ventilators (PPVs) appears relatively straightforward in terms of regulation, as these devices primarily operate in two states: idle (mainly after startup) and full power, characterized by generating the maximum airflow through the device [25]. This is to fulfil the primary function of the device, which is the effective ventilation of buildings. The intake of air by the fan rotor causes a change in air velocity in the engine area and a drop in pressure. This phenomenon was demonstrated by simulation studies conducted by Kaczmarzyk et al. in 2024 (Figure 1) [26]. Since carburetor control depends on air pressure values, its operation is based on the principles of fluid mechanics and uses pressure differences to regulate the flow of fuel and air. Considering that a classic internal combustion engine is designed to operate in an atmospheric air environment, undisturbed by significant pressure changes, it is reasonable to investigate whether such operating conditions affect the control of the air–fuel mixture.

The aim of this article is to study the lambda (λ) coefficient in controlling the air–fuel mixture in non-road small engines used in positive pressure ventilators, which cause changes in air pressure in the engine area compared to other applications of such engines. This research was conducted on commercial PPVs commonly used by fire services in the European Union [27].

2. Materials and Methods

The study involved four positive pressure ventilators (PPVs) commonly used by fire services in the European Union (Figure 2), whose characteristics are presented in

Table 1. Characteristics of tested fans.

Fan	W1	W2	W3	W4
Fan model	GX350	FOGO MW 22	GF210-20”	GX500
Manufacturer (city, country)	Ramfan (Spring Valley, NY, USA)	FOGO Sp. z o.o. (Wilkowice, Poland)	Taizhou Lion King Signal Co., Ltd. (Taizhou, China)	Ramfan (Spring Valley, NY, USA)
Power of the drive unit	4.1 kW	4.4 kW	5.1 kW	6.3 kW
Quantity of rotor blades	9	8	9	7
Flow straightener on the fan impeller	yes	no	yes	yes

(fan characteristics) and

Table 2. Characteristics of the combustion engine in the tested fans.

Fan	W1	W2	W3	W4
Engine model	GX160	750 series 163cc	GX270	GX270
Manufacturer (city, country)	Honda Motor Co., Ltd., Kumamoto Factory, (Kumamoto, Japan)	Briggs & Stratton Corporation, (Milwaukee, WI, USA)	Honda Motor Co., Ltd., Kumamoto Factory, (Kumamoto, Japan)	Honda Motor Co., Ltd., Kumamoto Factory, (Kumamoto, Japan)
Power of the drive unit	4.1 kW	4.4 kW	5.1 kW	6.3 kW
Displacement	163 cm3	163 cm3	196 cm3	270 cm3

(combustion engine characteristics). The selected models are equipped with drive units whose power is the most popular in terms of sales figures in the European Union. The engines were powered by gasoline [27]. The measurement system is shown in Figure 3.

The lambda (λ) coefficient was measured using the STAG AFR device from AC S.A. (Białystok, Poland). This device allowed for the determination of the composition of the air–fuel mixture feeding the internal combustion engine by measuring the oxygen content in the exhaust gases. The device set included a wideband BOSCH lambda probe 0 281 004 026 device from Robert Bosch GmbH (Bamberg, Germany). Additionally, the rotational speed of the fan rotor axis was measured using the Testo 470 tachometer from Testo (Titisee-Neustadt, Germany).

Additionally, the concentrations of CO₂, CO, and HC in the exhaust gases were measured using the ATAL AT 505 exhaust gas analyser from ATAL s.r.o. (Tábor-Horky, Czech Republic). The exhaust gas analyser is part of the Multi-Diag modular system, with the specifications of the analyser presented in

Table 3. Specifications of ATAL AT 505 portable exhaust emissions analyser.

Measured Parameter	Range	Resolution	Measurement Error
CO	0–10% vol	0.01% vol	0.03% vol
CO2	0–20% vol	0.01% vol	0.5% vol
HC	12–2000 ppm vol	1 ppm vol	10 ppm vol

.

Measurements were conducted in ten repetitions at five rotational speeds: idle at 1350 rpm, followed by increments of 500 rpm (1800 rpm, 2300 rpm, 2800 rpm), up to the maximum rotational speed for the full-power operation of the devices, which was 3450 rpm for W1, 3330 rpm for W2, 3290 rpm for W3, and 3585 rpm for W4, as recommended by the manufacturers.

According to the results, it was found that the data were not normally distributed. Consequently, a statistical analysis was performed to determine the confidence intervals of the mean, calculated based on ten repetitions. When determining the confidence intervals, Student’s t-distribution and a 95% probability level (p = 0.05) were used.

3. Results and Discussion

The results of this study, showing the lambda (λ) coefficient as a function of rotational speed, are presented in Figure 4. It can be observed that all tested positive pressure ventilators (PPVs) operate with very lean mixtures, ranging from 1.5 to 2.2. The typical range for a lean mixture is a lambda coefficient (λ) from 1.1 to 1.5. A higher value is unconventional for air–fuel mixture control. If the lambda coefficient (λ) is 2, this indicates a very lean air–fuel mixture. A lambda coefficient of 2 means that the actual air–fuel ratio (AFR) is twice the stoichiometric ratio. Such a lean mixture is rarely used in practise, as it can lead to ignition problems, high combustion temperatures, and potential engine damage [28].

Assuming that manufacturers use drive units designed for operation at atmospheric pressures appropriate for standard sea-level elevations, the airflow induced by the rotor disrupts the carburetor’s air–fuel mixture control, similar to how a carbureted engine operates at high altitudes or under low atmospheric pressure conditions, where the surrounding air pressure is lower. This causes a decrease in air density and consequently a reduction in the amount of air drawn into the carburetor [6].

The airflow induced by the fan rotor increases pressure on the discharge side and decreases pressure on the intake side. The engine located on the intake side of the rotor operates under low pressure conditions, which should manifest as lower atmospheric pressure and reduced fuel metering [26,29]. The carburetor operates based on the pressure difference between the atmosphere and the internal pressure in the carburetor (in the Venturi constriction). At lower atmospheric pressure, the pressure difference is reduced, resulting in less air intake and consequently less fuel intake. Carburetors are typically calibrated to provide the correct fuel-to-air ratio at standard atmospheric pressure (approximately 1013 hPa at sea level). At lower atmospheric pressures (at higher altitudes), this ratio is disrupted—less air also means less fuel, as the carburetor cannot draw in sufficient air to pull in the same amount of fuel as at normal pressure.

Lean mixtures tend to cause higher combustion temperatures. This can lead to engines overheating and, in extreme cases, damage to pistons, valves, and cylinder heads. An engine running on a mixture that is too lean may struggle to achieve full power. A fuel shortage means there is insufficient energy released during combustion, leading to reduced engine efficiency. Higher combustion temperatures can increase the risk of knock, which is the premature ignition of the mixture in the cylinders. Knocking can cause severe engine damage, including to pistons, piston rings, and cylinder heads. Lean mixtures can also be harder to ignite, leading to ignition issues, uneven engine operation, and decreased performance. Additionally, lean mixtures may result in higher emissions of nitrogen oxides (NO_x), which are harmful to the environment. Increased NO_x values in exhaust gases emitted by PPVs were observed by Warguła et al. in 2023 [30]. Elevated combustion temperatures promote the formation of greater amounts of NO_x. High temperatures and inadequate lubrication (due to insufficient fuel) can lead to the increased wear and corrosion of engine components, potentially shortening engine lifespan.

According to the authors, carburetors in drive units for PPVs should undergo additional adjustments due to the changes in air pressure caused by the airflow induced by the fan rotor on the intake side. Contrary to general assumptions, the pressure in front of the rotor is not close to atmospheric pressure; instead, a vacuum is created, which, as indicated by the lambda coefficient results, affects the carburetor’s control of the air–fuel mixture.

As internal combustion engines continue to develop, the use of three-way catalytic converters in small non-road engines may accelerate their degradation. The expected lambda coefficient during the full-power operation of these devices appears to be λ = 1 or slightly leaner for optimal operation.

This study has enhanced our understanding of air–fuel mixture control in small non-road engines with carburetor systems under the specific operating conditions created by PPVs. Further research should be conducted under precise operating conditions, as indicated by the analysis of Kaczmarzyk et al. in 2024 [26], who noted changes in flow speed in this area which are certainly related to pressure changes. This should be verified experimentally.

Additionally, carburetors could be adjusted for specific applications to drive PPVs, though there is no certainty that their factory settings will allow for accurate adjustments. In control tests, efforts could be made to isolate the influence of the air stream drawn in by the fan rotor in an attempt to modify the air shadow affecting the carburetor system to achieve atmospheric pressure.

It is also possible to test the hypothesis that the airflow induced by the fan rotor, when properly directed from the air intake channel to the combustion chamber, results in a greater volume of air being drawn in. However, in this scenario, the carburetor should respond to the air–fuel mixture control process. Yet, the amount of air delivered might exceed the carburetor’s fuel dosing capacity, resulting in a lean mixture.

Calculating carburetor fuel metering at different atmospheric pressures requires understanding how pressure changes affect air flow and the air–fuel mixture. Specifically, we are interested in how the mass of air delivered to the engine changes and, consequently, how the amount of fuel should be adjusted. The density of air at normal atmospheric pressure, for example, 1013.25 hPa (1), and the density of air at a lower atmospheric pressure, for example, 900 hPa (2), should be considered. Comparing the mass air flow rate at a lower pressure to normal pressure (3) reveals a decrease of about 10%. This necessitates an adjustment in fuel metering. At lower atmospheric pressures, the carburetor will meter less fuel to maintain the proper AFR. This is due to the reduced mass air flow, which requires the fuel quantity to be properly regulated to avoid a too lean mixture.

$${\rho }_{atm}={\displaystyle \frac{P_{air}}{R·T}}={\displaystyle \frac{101325\mathrm{P}\mathrm{a}}{287·293\mathrm{K}}}\approx 1.204{\displaystyle \frac{\mathrm{k}\mathrm{g}}{{\mathrm{m}}^3}}$$

(1)

where

${\rho }_{atm}$—atmospheric air density;

$P_{air}$—air pressure;

R—gas constant for air;

T—air temperature.

$${\rho }_{low}={\displaystyle \frac{P_{air}}{R·T}}={\displaystyle \frac{90000\mathrm{P}\mathrm{a}}{287·293\mathrm{K}}}\approx 1.084{\displaystyle \frac{\mathrm{k}\mathrm{g}}{{\mathrm{m}}^3}}$$

(2)

where

${\rho }_{low}$—air density at lower atmospheric pressure.

$${\displaystyle \frac{{\dot{m}}_{air,low}}{{\dot{m}}_{air,atm}}}={\displaystyle \frac{1.084}{1.204}}\approx 0.9$$

(3)

where

${\dot{m}}_{air,low}$—mass air flow rate at lower atmospheric pressure;

${\dot{m}}_{air,atm}$—mass air flow rate at atmospheric pressure.

According to the graph of the relationship between the fuel–air mixture’s composition and the emissions of CO, HC, and NO_x developed by Herner and Riehl in 2013, it is possible to estimate the increase or decrease in selected emissions relative to the stoichiometric mixture (Figure 5) [31].

The conducted research on CO₂, CO, and HC concentrations in the exhaust gases for all four fans (Figure 6—W1, Figure 7—W2, Figure 8—W3, and Figure 9—W4) showed correlations consistent with the theory presented by Herner and Riehl in 2013 (Figure 5) [31]. It can be observed that as the air–fuel mixture becomes leaner, the values of CO and HC decrease (Figure 6, Figure 7, Figure 8 and Figure 9). For instance, in the case of the tested air–fuel ratios for λ ≈ 1.55, the CO concentration ranges from 3.2 to 5.9 vol%, and HC ranges from 90 to 240 ppm vol, with these ranges being predominantly associated with PPV W3 (Figure 10 and Figure 11). In the λ range from 1.55 to 2.25, the CO concentration is from 0.9 to 5.9 vol%, and HC ranges from 40 to 325 ppm vol. The lowest concentrations of CO and HC are exhibited by PPV W4, regardless of the λ value (ranging from 1.88 to 2.15), with CO concentrations between 0.9 and 2.40 vol% and HC around 40 ppm vol.

The emission of hydrocarbons (HCs) and carbon monoxide (CO) in the exhaust gases of internal combustion engines results from incomplete fuel combustion. The causes of HC and CO emissions can be explained by incomplete combustion. Under ideal conditions, the fuel in the engine should completely combust into carbon dioxide (CO₂) and water (H₂O). However, in reality, the combustion process is not perfect, leading to the emission of HC and CO. When there is an excess of fuel in the air–fuel mixture (a rich mixture), the engine does not supply enough oxygen for the complete combustion of the fuel, resulting in more carbon monoxide (CO) and unburned hydrocarbons (HCs). On the other hand, in lean mixtures, the combustion temperature may drop, and when the temperature is too low, the fuel does not completely burn, leading to higher emissions of HC and CO.

Meanwhile, CO₂ emissions are directly dependent on the amount of fuel burned. An increase in engine load, which rises with the speed and a greater flow rate through the PPV, is associated with increased fuel consumption, and the CO₂ vol% value increases for all PPVs (Figure 6, Figure 7, Figure 8 and Figure 9). The concentration values generated by the PPV are consistent with the results of other researchers, which are within the range of CO₂ 2.22–6.98%, CO 1.28–3.57%, HC 46.13–135.56 ppm, NO_x 68.68–197.50%, and PM10 0.42–0.93 mg/m³, as measured by the Axion RS+ portable emissions measurement system (PEMS) from Global MRV under real operating conditions, i.e., with the PPV operating at full power [32].

Small non-road SI engines with similar engine displacement in other applications, such as wood chippers, are characterized by a λ coefficient close to 1, with a potential trend toward a slightly leaner air–fuel mixture [33,34]. The significant difference in the operating conditions of the engines used in PPVs results from the substantial air movement in the area surrounding the engine.

Studies of exhaust gases from internal combustion engines under real operating conditions often reveal further directions for the development of the machines, vehicles, or processes they implement. This trend has been demonstrated in research on passenger cars [35], trucks [36], combine harvesters [37,38], wood harvesters [39,40], and agricultural tractors [41].

Small non-road SI engines are a special category of engine in the European Union subject to regulations limiting their exhaust emissions (Regulation 2016/1628/EU [42]). This category prioritizes the functionality of devices equipped with these engines over the high standards for harmful exhaust gas emissions, focusing mainly on factors such as low weight, small device size, and simple and relatively inexpensive construction. A review conducted by Warguła et al. in 2018 of available internal combustion engines in this group showed the dominance of engines with carburetor fuel systems [43,44]. The most technologically advanced commercial power unit available in this group is Honda’s iGX series engines, featuring an electronically controlled carburetor throttle [2]. Solutions with fuel injection systems [10,45] or alternative fuels (within the meaning of the European Union Directive (2014/94/UE)) [7,33,46] are also available; however, these are mainly prototype solutions. Of course, in the future, the use of low-power and small-capacity engines from another engine category, subject to different exhaust emission standards and, consequently, higher technical advancements, such as motorcycle or moped engines [47,48,49,50], could be considered. However, these solutions are more expensive and not required by EU homologation regulations, so they are not commercially used by PPV manufacturers.

The issue identified in this article, regarding the improper control of the lambda coefficient (λ) in engines used for PPVs, may stem from significant changes in atmospheric pressure that PPV designers did not anticipate when commercially utilizing non-road engines. Potential solutions to improve carburetor performance under substantial atmospheric pressure changes could be found in designs intended for aircraft engines. Automotive and non-road carburetors differ from aviation carburetors due to their varying operating conditions, including significant differences in ambient air pressure. An automotive and non-road carburetor operates in conditions close to sea level, where atmospheric pressure variations are relatively minor. As a result, it is not necessary to account for large pressure changes in its design. In contrast, an aviation carburetor must function across a wide range of altitudes, where air pressure decreases significantly as altitude increases. To maintain the proper fuel–air mixture, aviation carburetors are equipped with mechanisms to compensate for atmospheric pressure drops, such as automatic mixture adjustment systems (e.g., an automatic main needle). Examples of such designs are described by Kittler in 1939 [51] and Mock in 1942 [52]. Another direction for development could be the adoption of a simple electronically controlled fuel injection system, which would solve most of the issues associated with carburetor systems, as has been done in the automotive [6,53] and aviation industries.

4. Conclusions

The specific operating conditions of small non-road SI engines used to power positive pressure ventilators (PPVs) result in these engines operating under a reduced pressure compared to atmospheric pressure. The fuel supply systems in these engines use carburetor-based systems, which depend on ambient pressure for their operation. This article aimed to investigate the lambda (λ) coefficient in terms of controlling the air–fuel mixture of commercial engines used for this application. It was found that all four drive units (the most popular models used by fire services in the European Union) operate at full power (with maximum airflow efficiency) with lean and very lean mixtures, with lambda coefficients ranging from 1.6 to 2.2. These mixtures are considered lean and very lean, which can lead to accelerated engine wear, increased emissions of harmful exhaust gases (primarily NO_x), and a reduced effective performance of the device. In this study, the observed changes in CO₂, CO, and HC concentrations aligned with our theoretical analyses, with the measured values ranging from 2.3 to 5.6 vol % for CO₂, 1 to 6 vol % for CO, and 1 to 7 ppm vol for HC. The study has contributed to our understanding of the air–fuel mixture control issues in PPV-driving engines under specific operating conditions (forced air movement causing reduced air pressure). The observed lambda (λ) values are confirmed by the research; however, further work is needed to investigate the causes of this operational state. The suggested cause, related to pressure changes, is evaluated based on CFD analyses, and the experiments should be repeated under experimental conditions. Additionally, further development is needed for the design of fuel supply systems to ensure correct lambda (λ) coefficients under these conditions.