Straight-three engine
A straight-three engine, also known as an inline-three engine or inline-triple (abbreviated I3 or L3), is a type of reciprocating piston internal combustion engine featuring three cylinders arranged in a single straight line along a common crankshaft.[1] This configuration allows for a compact design suitable for small to mid-sized vehicles, with the cylinders typically operating on a four-stroke cycle in automotive applications.[2] In operation, the straight-three engine follows a firing order of 1-3-2 or 1-2-3, resulting in a firing interval of 240 degrees of crankshaft rotation, which provides smoother power delivery than two-cylinder engines but introduces inherent challenges in vibration management.[1] While primary and secondary inertial forces are theoretically balanced due to the symmetric piston movements, the design generates rocking couple vibrations—end-to-end oscillations—that require countermeasures such as a single counter-rotating balance shaft to minimize noise, vibration, and harshness (NVH).[2] Modern implementations often incorporate turbocharging, direct fuel injection, and variable valve timing to enhance power output and efficiency, enabling displacements around 1.0 to 1.5 liters to deliver performance comparable to larger four-cylinder units while reducing weight and friction losses.[3] The straight-three configuration dates back to the early 20th century, with examples such as the 1905 Rolls-Royce 15 hp, and saw early adoption in two-stroke engines by manufacturers like Saab in the 1950s for models such as the Saab 93 and 96, and later in Japan's Kei car segment from the 1970s, exemplified by Suzuki's lightweight designs like the Cervo.[4] Its resurgence in the late 1990s and 2000s stemmed from downsizing trends for emissions compliance, highlighted by the 1998 Smart ForTwo and Ford's 2011 1.0-liter EcoBoost, which achieved high torque (170 Nm) and low consumption (under 5 L/100 km).[4] As of 2025, it powers diverse vehicles from efficient city cars like the Toyota Yaris and Ford Fiesta to high-performance models such as the Toyota GR Yaris (224 kW, 400 Nm), prized for its balance of compactness, fuel economy, and responsive handling.[4][5]Design
Cylinder Configuration
A straight-three engine, also known as an inline-three, is a piston engine featuring three cylinders arranged in a single straight line parallel to the crankshaft axis, sharing a common crankshaft.[6] This configuration distinguishes it from multi-bank designs, such as V-type engines, by aligning all cylinders on one side of the crankshaft.[7] In automotive applications, straight-three engines typically have displacements ranging from 0.5 liters to 2.0 liters, balancing compactness with sufficient power output for small to mid-size vehicles.[8] Bore and stroke ratios are tailored to the engine's goals, often slightly undersquare for better low-end torque or oversquare for higher revving; for instance, the Ford 1.0L EcoBoost uses a 71.9 mm bore and 82 mm stroke, resulting in a bore/stroke ratio of about 0.88 to prioritize efficiency and balance.[9] The crankshaft design incorporates three throws spaced at 120-degree intervals, enabling evenly spaced firing events every 240 degrees of rotation in four-stroke operation, which supports smoother power delivery compared to uneven configurations.[10] The inline layout enhances packaging efficiency by minimizing overall width, making it ideal for transverse mounting in front-wheel-drive setups where space is limited, unlike wider V-engine alternatives.[7] It also promotes balanced weight distribution longitudinally along the vehicle chassis and simplifies cooling through a unified cylinder bank, allowing more straightforward coolant circulation than in multi-bank engines with separate heads.[11]Balance and Vibration
Straight-three engines configured with a 120-degree crankshaft spacing exhibit perfect primary balance, where the inertial forces from reciprocating masses cancel out completely due to the symmetric arrangement of the three crankpins.[12] This layout ensures that the vector sum of the primary forces (proportional to the piston mass, crank radius, and squared angular velocity) is zero at all times.[12] Similarly, the secondary balance is inherently perfect, as the second-order forces—arising from the non-sinusoidal motion of the pistons—are also symmetrically balanced, eliminating vibrations at twice the crankshaft speed from these components.[12] Despite this favorable force balance, the inline arrangement introduces a rocking couple due to the longitudinal offset of the cylinders along the crankshaft axis. The central cylinder's reciprocating motion is aligned with the engine's centerline, but the outer cylinders generate equal and opposite moments about this axis, resulting in an unbalanced rocking torque that manifests as vertical vibrations. This primary rocking couple occurs at engine speed, while the secondary rocking couple produces vibrations at twice engine speed, with the latter often more perceptible in operation.[12] The torque tends to rock the engine block end-to-end, with intensity depending on engine dimensions and speed. To mitigate these rocking couples, manufacturers commonly employ balance shafts or optimized crankshaft counterweights. A balance shaft, typically counter-rotating at twice crankshaft speed, generates an opposing torque to neutralize the vibrations; for instance, the BMW B38 1.5-liter inline-three engine in the MINI Cooper integrates a counter-spinning balance shaft beneath the crankshaft to cancel both primary and secondary couples effectively.[13] Alternatively, counterweights on the crankshaft can partially offset the imbalance by adjusting the rotating masses, as seen in various production designs where they reduce the net moment without additional shafts.[14] Some engines, like the Ford 1.0-liter EcoBoost, forgo balance shafts in favor of precisely weighted components on the front pulley and rear flywheel, along with hydraulic engine mounts, to redirect and absorb residual vibrations while minimizing added weight and complexity.[15]Firing Order
In straight-three engines operating on the four-stroke cycle, the standard firing orders are 1-2-3 or 1-3-2, resulting in power strokes spaced at even 240-degree intervals of crankshaft rotation, as the full cycle spans 720 degrees divided among three cylinders.[2] This even spacing ensures consistent ignition timing relative to the crankshaft's position, promoting balanced exhaust gas flow and combustion efficiency. Even firing in a straight-three configuration equates to a 120-degree phase shift between cylinders when viewed in terms of equivalent crankshaft throw angles, contrasting with uneven alternatives like clustered firings that could lead to irregular torque delivery. To illustrate, consider a typical 120-degree crankshaft where pistons are offset: at 0 degrees, cylinder 1 is at top dead center (TDC) firing; cylinder 2 reaches TDC at 240 degrees; and cylinder 3 at 480 degrees, completing the cycle at 720 degrees back to cylinder 1. This rotation can be visualized as:- 0°: Cylinder 1 at TDC (power stroke begins), cylinders 2 and 3 at intermediate positions (120° and 240° from TDC).
- 240°: Cylinder 2 at TDC (fires), cylinder 1 descending post-power, cylinder 3 approaching TDC.
- 480°: Cylinder 3 at TDC (fires), others in overlap phases.